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                  UNDERSTANDING SOLAR TERRESTRIAL REPORTS
                PART I - MORPHOLOGICAL ANALYSIS OF PHENOMENA
                                REVISION 1.2



                                  _A_B_S_T_R_A_C_T

              This document is intended to aid those who are
         interested in interpreting and using the material
         presented in the various solar terrestrial reports that
         are posted over the networks.  This document has been
         written under the assumption that the reader is unfamiliar
         with the terrestrial impacts of solar-related activity.
         It is therefore not intended for those who already have a
         knowledge of solar physics and/or geophysics.  Some of the
         terms contained in this document are undefined.  For
         definitions of undefined terms, request the "Glossary of
         Solar Terrestrial Terms" from "[e-mail]".

              There are two parts to this document, split into two
         completely separate sections.  This first part describes
         the morphology of solar and terrestrial phenomena.  Part I
         is fairly extensive and is intended to give the reader
         enough background knowledge to understand, interpret and
         apply the information presented in part II.  Part II
         discusses the format and proper interpretation of the
         solar terrestrial reports.  They should be read in proper
         sequence in order to be best understood.



   July 14, 1991

































                  UNDERSTANDING SOLAR TERRESTRIAL REPORTS
                PART I - MORPHOLOGICAL ANALYSIS OF PHENOMENA
                                REVISION 1.2



   _1.  _I_n_t_r_o_d_u_c_t_i_o_n

        In March of 1989, some spectacularly powerful  solar  terrestrial
   events occurred.  An very complex and active solar region erupted with
   almost unprecedented levels of activity.  Flare activity broke records
   that were held for over 30 years.  Intensely severe geomagnetic storm-
   ing induced electrical currents in powerlines,  which  resulted  in  a
   total  collapse  of  the  Hydro Electric power distribution network in
   Quebec.  This resulted in the loss of electrical power for over 6 mil-
   lion  Canadians.   Telecommunications  equipment  experienced powerful
   voltage surges on the power supply lines of transatlantic  fiber-optic
   cables in excess of 700 volts.  Oil pipelines experienced rapid strong
   variations in pipe-to-soil potentials, producing  electrolytic  corro-
   sion at flaws in the pipeline coating.  Radio propagation was severely
   effected by both strong PCA activity and severe geomagnetic  storming.
   HF  radio  signals  were completely blacked out over many global loca-
   tions, and remained at  very  poor  levels  for  at  least  24  hours.
   Auroral  activity was easily visible as far south as Florida (and even
   further).  Some satellites, unable to withstand the rapid fluctuations
   in solar wind pressure, began tumbling out of control.

        It is well known that solar activity has an astonishing influence
   on  terrestrial  Earth-based  systems.   The events of March 1989 will
   long be remembered as a prime example of the power and  influence  the
   sun can have on our environment and activities.

        The solar terrestrial reports and associated alerts/warnings have
   been  posted  over  the  networks in order to aid in the prediction of
   terrestrial conditions which might be expected from  solar  and  other
   related  activity.   This  document  is  intended  to help explain the
   nature of these various reports so that application of the  data  con-
   tained therein can be properly applied to the various fields which can
   be affected.

        Part I of this document will examine  the  basic  physics  behind
   such  solar  phenomena as sunspots, flares, and coronal holes in terms
   that should be easily understood by the  layman.   Following  this,  a
   basic  overview  of  the  geomagnetic  field and some of its important
   features will be discussed.  The characteristics of radio wave  propa-
   gation  will  then  be  explored  for  VLF, HF and VHF signals as they
   relate to geomagnetic  and  auroral  activity.   Following  this,  the
   characteristics and behavior of auroral activity will be considered in
   conjunction with astronomical observations and magnetic  fluctuations.
   Concluding  this  section will be a discussion on the impact of severe
   geomagnetic storms.  This  discussion  will  include  the  effects  of



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                                   - 2 -


   strong magnetic storming on such environments as electrical power dis-
   tribution networks, atmospheric circulation, and radio communications.

        Part II of this document will delve into the format of the  solar
   terrestrial reports.  The proper interpretation of the predictions and
   various charts contained in the weekly Solar Terrestrial Forecast  and
   Review will be discussed. An examination of the flare alerts and warn-
   ings will then be conducted, followed by an analysis  of  the  geomag-
   netic  and auroral storm alerts which are posted when necessary.  Con-
   cluding this section will be a brief overview of the material  covered
   in  parts I and II of this document.  Hopefully, this document will be
   cohesive and interesting enough to be of value to those who are  seri-
   ous  about examining the relationships between solar activity and ter-
   restrial impacts.

   _2.  _C_h_a_r_a_c_t_e_r_i_s_t_i_c_s _o_f _t_h_e _S_u_n

        The sun is a dynamic, complex object that we are only now  begin-
   ning  to understand.  It has been a source of study and wonderment for
   centuries.  Although many questions have  been  raised  regarding  its
   influence  on  the  Earth,  aside from the fact that it is our primary
   source of energy, only during the past century have real  achievements
   been  made  toward understanding its intricate nature and influence on
   our environment.

        We now know, for example, that the sun has regular,  fairly  con-
   stant  cycles.  Through persistent observations and meticulous record-
   keeping, we know that the sun runs through  cycles  of  activity  with
   periods of about 11 years.  We know that the sun also has a longer, 22
   year cycle in which the magnetic polarity of the solar poles  actually
   reverse  sign.   We  know  that the sun is a rotating sphere that com-
   pletes one revolution approximately once every 27-28  days.   We  also
   know  that  areas near the solar poles rotate at slower velocities and
   therefore take longer to complete one revolution than areas  near  the
   solar equator (this has been termed "_d_i_f_f_e_r_e_n_t_i_a_l _r_o_t_a_t_i_o_n").

        It has long been known that visibly dark regions often plague the
   surface  of  the sun.  The ancient Chinese noticed these dark spots at
   least 15 centuries ago.  Early solar astronomers noticed,  over  time,
   that  the number of spots observed on the sun vary in cyclic patterns.
   They also noticed that the number of aurorae that were seen were posi-
   tively correlated with the number of sunspots observed on the sun.

        It wasn't until the first satellites investigated the  domain  of
   space, that we began to realize the intricate nature and varying forms
   of activity that occur on the sun.  We discovered and studied what  is
   called  the  solar corona, a great expanse of superheated rarefied gas
   extending outward many solar radii from the solar surface.   We  exam-
   ined  in  great detail the morphology of solar flares, one of the most
   powerful natural explosions known to  man.   Our  investigations  have
   revealed  a  great abundance of radiations emitted by the sun, much of
   which  is  filtered  out  by  our  terrestrial  atmosphere.   We  have
   developed  new  techniques  of  studying  the sun at different optical
   wavelengths, which has given us a wealth of new information  regarding



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                                   - 3 -


   the  physics  of  phenomena  seen  on the sun.  We have witnessed many
   forms of activity: prominences, filaments, plages, faculae,  granules,
   and many other forms of activity.

        The first step in understanding the relationships  between  solar
   activity and terrestrial phenomena is to obtain a knowledge of some of
   the basic characteristics of the sun and its attendant  activity.   In
   this  first  section,  we  will  attempt  to  cover enough material to
   explain some of the properties and relationships required  for  strong
   interactions between the sun and the Earth.

   _2._1.  _S_u_n_s_p_o_t_s _a_n_d _t_h_e _S_o_l_a_r _F_l_u_x

        Galileo is credited as being the first person  to  discover  sun-
   spots  telescopically  around the year 1610.  He immediately noted the
   presence of black spots on the bright surface of the sun.  It was also
   observed that these spots moved across the surface of the sun and gra-
   dually changed shapes from day to  day.   These  spots  puzzled  solar
   astronomers  for  years  and  resulted  in  some interesting, although
   incorrect, theories regarding their origin.

        Through time, we have developed better instruments to resolve the
   features  of  sunspots.   This  has increased our factual knowledge of
   sunspots which has allowed us to refine our theories to model sunspots
   more accurately.

        We now know that sunspots are cool regions of the sun.  They  are
   regions approximately 2,000 Kelvin cooler than the surrounding surface
   of the sun.  The suns surface (called the _p_h_o_t_o_s_p_h_e_r_e), is about 5,800
   degrees  Kelvin.   The  cooler  temperatures  within sunspots are what
   cause them to appear darker  than  the  surrounding  photosphere.   In
   actuality,  a  sunspot separated from the sun and placed in the black-
   ness of space would radiate a great deal of energy, and  would  appear
   as  an  intense  source  of bright white light.  It is only due to the
   contrasting temperature of the suns surface  that  cause  sunspots  to
   appear dark.

        The dark central portion of a sunspot is called the _u_m_b_r_a and  is
   most  often  associated  with  a less dark region called the _p_e_n_u_m_b_r_a.
   Sunspots vary greatly in size and complexity, but are generally around
   37,000  kilometers in diameter.  Sunspots almost always form in groups
   of two or more.

        Sunspots are regions of intense magnetic  fields.   The  magnetic
   fields originate from deep within the sun and gradually propagate out-
   ward.  When they reach the surface, they cause the  gases  within  the
   intense core of the magnetic field to cool.  Magnetic fields forming a
   sunspot often curve around and re-enter  the  sun  at  another  nearby
   location.   At each point where the magnetic field enters or exits the
   sun, a sunspot is formed.

        Sunspots rotate in the same direction as the sun, and at the same
   (or  nearly  the  same) speed as the surrounding gases.  Sunspots near
   the solar poles therefore take longer to complete one revolution  than



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   do  spots  near  the  solar equator.  Sunspots generally take about 27
   days  to  complete  one  revolution  near  the  solar  equator.   This
   increases  to  over  35 days for sunspots existing at high solar lati-
   tudes.

        Sunspots have distinct lifecycles.  Although the  lifetime  of  a
   single spot can extend for many weeks (sometimes months), the activity
   cycle of sunspots is very distinct.  Sunspots begin  as  small  specks
   (called  "_p_o_r_e_s")  which  often grow rapidly into larger more distinct
   spots.  As they grow in size, they  develope  penumbral  regions  sur-
   rounding  the dark umbral core.  Nearby, other sunspots often form and
   grow simultaneously.  Unlike the cooler temperatures within the  spots
   themselves,  the  outer regions of the spots (outside of the penumbral
   zones) are often superheated and appear brighter than the rest of  the
   photosphere.  These brighter regions, called _f_a_c_u_l_a_e mark the presence
   of strong magnetic fields near the surface of the sun.  These are  all
   characteristics of a maturing sunspot group.

        As a sunspot group ages, the spots  within  the  group  begin  to
   spread  apart  and  drift away from what was once a compact cluster of
   spots.  This spreading is caused by the drift of the  associated  mag-
   netic  fields  away from the central spawning region.  As the magnetic
   fields drift, they often decrease in intensity and diffuse into weaker
   regions.   Sunspots begin to fade away and disappear.  Eventually, all
   of the sunspots fade away and die, leaving only a  brighter  patch  of
   faculae  marking  the  region  that once was an active sunspot forming
   region.  Over time, even the faculae disappear, leaving no trace  that
   sunspots once existed over that region.

        This life cycle is apparent in many sunspot  groups  that  forms.
   However,  not all sunspot groups behave this way.  Some groups of sun-
   spots never reach full maturity before perishing.  Others may  sustain
   mature  configurations  for  weeks  before  beginning to show signs of
   decay.  And still others may exhibit multiple  cycles  of  growth  and
   decay  before  finally  dying.   Although  the  morphology of sunspots
   varies dramatically, the general life  cycle  above  applies  in  most
   cases.

        Sunspots are sources of enhanced radiation  emissions.   For  the
   person  dependent  on long-distance radio communications, sunspots can
   help provide the enhanced radiation  necessary  to  provide  excellent
   radio conditions over long-distance signal paths.

        The radiation emitted by sunspots  are  most  often  concentrated
   within  groups of sunspots lying in relatively close proximity to each
   other.  The radiation covers a  host  of  different  wavelengths  from
   Gamma  rays all the way down to radio-waves.  The radiation emitted by
   all of the sunspot groups visible on the sun are measured by a variety
   of  instruments.   Satellites  constantly monitor the radiation levels
   from the sun which cannot be measured from  the  ground  (due  to  the
   filtering  effect  of  the  Earths  atmosphere).  Radiation which does
   reach ground levels are measured by sensitive radio receivers tuned to
   those wavelengths.




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                                   - 5 -


        One of the wavelengths of  radiation  which  does  penetrate  the
   Earths  atmosphere  down to ground-levels is the 2800 MHz band (or the
   10.7 centimeter wavelength band).  This intensity of  noise  (ex.  the
   intensity of the radiation) emitted from the sun on this wavelength is
   measured daily by the Algonquin Radio Observatory  in  Ottawa  Canada.
   The  intensity  measurements obtained from this observatory are broad-
   cast world-wide by radio station WWV (and all other related  stations)
   in the form of a _s_o_l_a_r _f_l_u_x.  This solar flux represents the intensity
   of the solar radiation being measured at the Earths surface  from  the
   sun.

        The solar flux is fairly critical in radio  communications  work.
   It  has been found that the radiation intensity at 2800 MHz correlates
   fairly well with the ionization levels at altitudes sensitive to  high
   frequency (HF) radio communications.  High solar flux values generally
   translate into better radio communications.  It also generally marks a
   period  of  better  long-distance communications using higher frequen-
   cies.  The maximum usable frequency (MUF) during periods of high solar
   flux often exceed 50 MHz, providing long-range communications capabil-
   ities for operators using very high frequencies (VHF).

        The solar flux (and hence, the radiation intensity from  the  sun
   at  2800 MHz) is very dependent on the number of sunspots.  Large sun-
   spot groups can produce steep increases in the solar flux.  Solar flux
   values  in  excess of 300 are indicative of extensive sunspot activity
   and may coincide with very good long-range communications  on  HF  and
   perhaps  even  lower VHF frequencies.  Low solar flux values below 100
   are usually indicative of periods where very little  sunspot  activity
   is visible on the sun.  The solar flux is therefore an excellent means
   for monitoring sunspot activity.  Increases in solar flux indicate the
   emergence  or  growth  of sunspot areas on the sun, while decreases in
   the solar flux indicate the disappearance or death of sunspots on  the
   sun.

        Since the sun rotates with a period averaging approximately 27-28
   days,  it  is reasonable to question whether or not the number of sun-
   spots visible on the sun fluctuate with a period of around 27-28 days.
   This  is in fact, true.  The rotation of the sun often causes sunspots
   to rotate out of view and then reappear on the opposite  side  of  the
   sun  about  14 days later.  We say 14 days later because by the time a
   sunspot group has rotated out of view, it has already  completed  half
   of  its rotation period.  So it only takes 14 days for a sunspot group
   to rotate to the opposite side of the sun and back into view.

        This cyclic behavior is also manifest in the solar flux.  Because
   the  solar  flux  is  dependent  on sunspot activity, the value of the
   solar flux often fluctuates in tandem with  sunspot  activity.   As  a
   sunspot  group  rotates out of view, the solar flux decreases in value
   (sometimes dramatically if the sunspot group is extensive).   Approxi-
   mately  14 days later, the same sunspot group may (assuming it doesn't
   die) rotate back into view on the opposite side of the  sun,  with  an
   attendant increase in the solar flux.

        This cyclic pattern can be easily seen when  the  solar  flux  is



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   plotted  over  time.   The Solar Terrestrial Forecast and Review plots
   the solar flux graphically over a period of 60 days.  By observing the
   cyclic  pattern, it is relatively easy to determine approximately when
   the next peak will occur.  Using this  information,  enhanced  general
   radio  conditions can also be predicted with relatively good accuracy.
   As will be seen in later sections, however, the quality of radio  con-
   ditions depends on much more than simply the solar flux.

   _2._2.  _T_h_e _S_u_n_s_p_o_t _C_y_c_l_e

        Just as the number of sunspots fluctuate  with  periods  of  near
   27-28  days,  the  sun  exhibits  a longer period cycle which directly
   effects the population of sunspots that form over the  entire  surface
   of  the  sun.   This cycle has been called the _s_u_n_s_p_o_t _c_y_c_l_e since the
   primary effect of the cycle is on sunspot activity.

        To discern this longer cycle, it is necessary to plot the  number
   of  sunspots  observed  on  the  surface of the sun persistently for a
   period of about 11 years.  If this is done, it becomes  apparent  that
   the  number  of  sunspots  which form and become visible decrease to a
   minimum over a period of about 6 to 8  years,  followed  by  a  fairly
   rapid  increase  to  a peak over a period of about 3 to 5 years.  This
   cyclic behavior represents the sunspot cycle.

        The solar flux likewise follows an 11 year cycle.  But since  the
   solar flux represents (at least in part) the quality of radio communi-
   cations (ex.  distance and stability of communications), radio commun-
   ications  also  follow a cyclic pattern that is in phase with the sun-
   spot cycle.

        There are many other aspects of solar activity which closely fol-
   low  the sunspot cycle.  These other forms of activity will be covered
   in later sections.

   _2._3.  _T_h_e _2_2 _Y_e_a_r _S_o_l_a_r _C_y_c_l_e

        Superimposed on the 11 year solar cycle is yet another cycle with
   a  period  of  about  22  years.   This cycle is primarily magnetic in
   nature and can be seen only by observing the  polarity  of  the  solar
   poles.

        The sun has an extensive magnetic field  which  reaches  far  out
   into  interplanetary  space.  If a compass were held while standing on
   the sun, the compass needle would deflect and point towards the  north
   and  south solar poles just as it does for us here on Earth.  However,
   unlike the Earth, the suns magnetic poles  reverse  polarity  approxi-
   mately  once  every 11 years for a total period of 22 years.  That is,
   once every 11 years, a person standing on the sun with a compass would
   notice  the  needle reversing directions.  Another 11 years later, the
   direction of the compass needle would reverse directions again, point-
   ing  back  in  the  same  direction  as  it  originally did when first
   observed.  The characteristics of  this  cycle  were  first  noted  by
   Hale[1] and Hale and Nicholson[2].  It is a fairly important cycle  as
   _________________________



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   will be explained below.

        As was seen in section 2.1, sunspots  are  intimately  linked  to
   magnetic  fields.  This 22 year cycle affects the polarity of the sun-
   spots that are formed in the northern and southern solar  hemispheres.
   It  also  affects  the  polarity  of the interplanetary magnetic field
   which is detected and measured from space by spacecraft.

        Near the minimum of each solar cycle, the polarity of  the  solar
   magnetic poles reverse sign.  This does not occur suddenly.  It can be
   a rather slow process.  Often, the solar poles become the same  polar-
   ity  before  the  full  reversal process completes.  When the northern
   solar hemisphere has a northern-magnetic pole, sunspots which form  in
   that  hemisphere  have  opposite  magnetic characteristics to sunspots
   which are formed in the southern hemisphere.  After the poles  reverse
   magnetic  polarity, the sunspots which form in the northern hemisphere
   likewise reverse magnetic characteristics.

        This cycle is important because it affects almost all of the mag-
   netic  characteristics  of  the sun as a whole and requires changes in
   the way we examine sunspot groups and their behavior.

   _2._4.  _T_h_e _S_o_l_a_r _A_t_m_o_s_p_h_e_r_e

        The suns atmosphere can be divided into three  distinct  regions,
   or  layers of differing physical properties.  Each of these layers are
   very important to those who expect to understand the  phenomena  which
   occur  within  the  various  regions.  We will very briefly review the
   properties and characteristics of these three regions, and  will  note
   the  types of phenomena which occur in the various layers of the solar
   atmosphere.

   _2._4._1.  _T_h_e _P_h_o_t_o_s_p_h_e_r_e

        The solar photosphere is the lowest  layer  of  the  solar  atmo-
   sphere.   This layer resides between 200 km and 400 km deep.  The pho-
   tosphere is responsible for contributing most of  the  light  that  we
   receive  here  at the Earth.  It is the photosphere which produces the
   so called _l_i_m_b _d_a_r_k_e_n_i_n_g _e_f_f_e_c_t, where the radiation intensity emitted
   from  the  sun decreases from the center of the solar disk to the edge
   (or limb) of the sun.  As we look closer to the  limbs,  our  line  of
   sight  approaches  tangency  to the solar sphere and therefore travels
   through a greater volume of the upper  photospheric  layers.   Because
   light from the deeper layers cannot reach us from the limbs due to the
   thickness and absorbing characteristics of the photosphere, we do  not
   see  as  deeply  into the photosphere when we look at the limbs, hence
   the limbs appear darker than does the central solar disk.
   _________________________
     [1] (1908) On the probable existence of magnetic fields in sunspots.
   Journal of Astrophysics #28, pg. 315-343.
     [2] (1925) The law of sunspot polarity. Journal of Astrophysics #62,
   pg. 270.




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                                   - 8 -


        As one would expect, the temperature  of  the  solar  photosphere
   increases with increasing depth.  In general, the effective solar pho-
   tospheric temperature is calculated (using Stefan's law) to  be  about
   5780 degrees Kelvin.  The photosphere represents the coolest region of
   the sun.  The temperature increases as you look deeper into the photo-
   sphere, and it also increases as you travel outward away from the pho-
   tosphere.

        The average density of the photosphere is relatively small;  even
   smaller than the density of the Earths atmosphere.  In fact, the aver-
   age density of the photosphere is only about one  thousandth  that  of
   the  Earths  atmosphere,  yet we can only see to a very small depth in
   the photosphere due to the high absorption and continuous spectrum  of
   radiation which is emitted by the photosphere.

        The photosphere is not a particularly  smooth  surface.   Through
   observations  using  powerful telescopes, plumes of rising and falling
   gas in the photosphere have been found.  These _g_r_a_n_u_l_e_s  can  be  seen
   over  the  entire  surface  of  the photosphere and range in size from
   about 200 km to over 1800 km.  Their average size  is  about  700  km.
   They  are  not  a long-lived phenomena.  Average liftimes for granules
   are only about 8 to 9 minutes.

        Sunspots as seen with the naked eye, are viewed as they appear on
   the  photospheric  layers  of  the sun.  Their domain, however, is not
   restricted to the photosphere.  Indeed, they can have profound effects
   in the suns chromosphere as well (discussed below).

   _2._4._2.  _T_h_e _C_h_r_o_m_o_s_p_h_e_r_e _a_n_d _S_p_i_c_u_l_e_s

        Immediately above the photosphere lies the chromosphere, an  area
   of the suns atmosphere where solar flares originate.  The chromosphere
   is much thicker than the photosphere.  It resides  between  the  upper
   surface  of  the  photosphere and extends to about 12,000 km above the
   surface of the photosphere.  There are basically three regions of  the
   chromosphere  which are defined according to the temperature stratifi-
   cation which occurs in that region.  The lower layer  of  the  chromo-
   sphere  extends  to  an  altitude  of about 1000 km.  The middle layer
   extends from 1000 km to about 4000 km, while  the  upper  chromosphere
   extends  from  4000  km  to  about  12,000  km.  Temperatures increase
   rapidly from the lower chromosphere to the upper chromosphere.  At the
   upper edge of the chromosphere, the temperature can increase to values
   in excess of 100,000 degrees Kelvin.

        The chromosphere is the home of another type of phenomena, called
   the  _s_p_i_c_u_l_e.   Spicules,  when  viewed  at  the  solar  limb using an
   appropriate monochromatic filter (such as an H-alpha  filter),  appear
   as  grass-like protuberances that project against the black background
   of space.  They occur primarily in the upper middle and  upper  layers
   of the chromosphere.

   _2._4._3.  _T_h_e _C_o_r_o_n_a _a_n_d _C_o_r_o_n_a_l _H_o_l_e_s

        The highest and most diffuse region of the  solar  atmosphere  is



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   known  as the _c_o_r_o_n_a.  This is a region of very low density gases that
   are superheated to exceedingly high temperatures.  It can only be seen
   during  a total solar eclipse, or by using a special instrument called
   a _c_o_r_o_n_o_g_r_a_p_h which automatically occults the bright  solar  disk,  in
   effect, simulating a total solar eclipse.

        The solar corona can only be seen when the bright surface of  the
   sun is completely blocked out.  The low density of the corona inhibits
   its ability to give off light, hence its surface brightness is only  a
   few millionths that of the suns disk.

        The corona has no well defined upper boundary.  When viewed using
   a  coronograph or during a total solar eclipse, the corona can be dis-
   cerned to distances in excess of  several  solar  radii.   Indeed,  it
   extends to great distances in space, out to a distance of several mil-
   lion kilometers, where it gradually becomes the solar wind.

        Whereas the temperature of the photosphere is only about 5800  K,
   the  temperature in the solar corona soars to values in the range of 1
   to 2 million degrees Kelvin.  Pressure waves propagating outward  from
   the  suns  convenctive  zone  in  lower levels provide the energy that
   heats the suns tenuous coronal regions to  such  extraordinarily  high
   temperatures.

        The solar corona exhibits several forms of activity.  When viewed
   using  a coronograph, bright transient features become visible.  These
   bubble-like projections called _s_o_l_a_r _t_r_a_n_s_i_e_n_t_s or _c_o_r_o_n_a_l _m_a_s_s  _e_j_e_c_-
   _t_i_o_n_s  (CMEs)  are  relatively  short-lived and expand rapidly outward
   through the corona.  These  disturbances  are  associated  with  radio
   bursts that are observed here on Earth.

        The high temperatures in the solar corona are sufficient to  pro-
   duce copious amounts of x-ray radiation.  This wasn't discovered until
   the early 1970s when the Skylab mission revealed the intricate  nature
   of the solar corona.  When viewed at x-ray wavelengths from space, the
   inner solar corona appears blotchy, with many bright points and exten-
   sive  areas  where  very little x-ray radiation appears to be emitted.
   These areas devoid of x-ray emissions, are called _c_o_r_o_n_a_l  _h_o_l_e_s.   It
   has  been  found  that  the passage of these coronal holes through the
   central solar meridian are almost always followed within 3 to  5  days
   by increased geomagnetic activity here on earth.  It is now known that
   these coronal holes are regions where the magnetic  field  lines  from
   the  sun  are open (ie. they don't immediately curl around back to the
   sun, but instead escape into interplanetary space).  Since the charged
   particles  in  the  sun naturally follow the magnetic field lines, the
   charged particles are allowed to escape into interplanetary space when
   the  magnetic  field  lines  of  the  sun  are open.  For this reason,
   coronal holes are often locations where escaping high-speed streams of
   charged  particles  from  the sun are allowed to impinge on the Earths
   space-environment, causing increased geomagnetic activity  and  occas-
   sional magnetic storms.

        Coronal holes most often reside near the solar poles,  where  the
   magnetic  field  lines  extend radially out into interplanetary space.



                               July 14, 1991





                                   - 10 -


   It is believed that the density of  charged  particles  and  also  the
   speed of the solar wind are increased over these regions.  The Ulysses
   space mission will hopefully  confirm  these  theories.   The  Ulysses
   spacecraft  is  on  its  way to Jupiter, where it will use the massive
   gravitational pull of the planet to slingshot the spacecraft  at  high
   velocities  in  an  orbit  that  will carry it over the solar poles to
   measure many aspects of the environment there.  The solar  poles  have
   never been seen before in any great detail.  Moreover, we are not able
   to determine the characteristics of space over the solar poles,  since
   the  orbit  of  the  earth  never carries us beyond solar latitudes in
   excess of about seven degrees on either side  of  the  solar  equator.
   Hence,  there  is  a significant amount of interest among solar physi-
   cists with regards to this mission.

        Near the solar poles, coronal holes  do  not  affect  the  earth.
   Their  primary  effects  propagate  well to the north and south of the
   Earths orbital plane.  Not until the coronal holes migrate toward  the
   solar  equator  do  we  begin  to notice the effects of coronal holes.
   When coronal holes migrate to solar latitudes below  approximately  30
   to  40 degrees, the relatively high speed streams of charged particles
   which emanate from these regions are able to begin to  impact  on  our
   terrestrial environment.

        Coronal holes rotate with the sun.  They are therefore capable of
   producing recurrent activity each time they rotate around the sun.  As
   they rotate, they change their form.  Sometimes they expand  in  size.
   Sometimes they contract and disappear.  During periods of sunspot max-
   imum, their forms change rapidly and their recurrent effects diminish.
   The numerous active regions which plague the surface of the sun during
   sunspot maximum are blamed for the rapid changes in  form,  appearance
   and  death  of  coronal holes. Coronal holes formed during the sunspot
   minimum, however, are often long-lived and may  last  for  many  solar
   rotations  before  they  finally  fade away or migrate back toward the
   solar poles.  During these  periods,  recurrent  geomagnetic  activity
   becomes well established.

   _2._5.  _F_o_r_m_s _o_f _S_o_l_a_r _A_c_t_i_v_i_t_y

        Among the various forms of solar  activity  are  plages,  facula,
   prominences,  filaments  and  the  powerful  explosions known as solar
   flares.  All of these forms of  solar  activity  are  associated  with
   active regions (sunspots).  However, their manifestations and trigger-
   ing mechanisms vary considerably.

        In the next several sections, we will briefly examine some of the
   properties  of  these  phenomena,  concentrating  most  heavily on the
   aspects of solar flares, erupting prominences and  disappearing  fila-
   ments, which have the most profound effects on the earth.

   _2._5._1.  _P_l_a_g_e_s _a_n_d _F_a_c_u_l_a_e

        The terms _p_l_a_g_e and _f_a_c_u_l_a are often used synonymously.  In fact,
   Deslandres originally introduced the words with the phrase _p_l_a_g_e _f_a_c_u_-
   _l_a_i_r_e. Since then, the  terms  have  evolved  into  two  similar,  yet



                               July 14, 1991





                                   - 11 -


   separate phenomena.  The term _f_a_c_u_l_a_e is now used to denote the bright
   regions seen in white light surrounding sunspots (as is  noticed  when
   sunspots  are  viewed  near  the solar limbs).  Faculae are therefore,
   _p_h_o_t_o_s_p_h_e_r_i_c phenomena.  Plages, on the other hand  are  _c_h_r_o_m_o_s_p_h_e_r_i_c
   phenomena  and can only be observed when viewed through an appropriate
   monochromatic light filter (such as an H-alpha filter).

        Plages and faculae are not separate phenomena.  Rather, they  are
   the  same  phenomena  manifested  at  different altitudes in the solar
   atmosphere.  Faculae may therefore be considered to  extend  into  the
   chromosphere,  where  the same phenomena is witnessed as chromospheric
   plage.

        As a general rule, the  plage  outlives  its  associated  facula,
   often  by  several  weeks.   Both types of activity form around active
   regions and can extend to quite  large  distances  around  the  active
   region.   Plage  and  faculae  do not extend as far north and south as
   they do  east  and  west.   Their  east-west  extensions  cause  their
   apparent  shapes  to  become  elongated.  They typically follow in the
   steps of the active regions and are always the last optical  phenomena
   to disappear when an active region dies.[3]

        Faculae contain bright granules which combine to form coarse mot-
   tles having diameters of about 5,000 km.  These mottles tend to string
   together into chains.  These chains of mottles are  what  compose  the
   faculae.   The  temperature in the upper photosphere where the faculae
   form is higher than the surrounding photosphere.  Also,  the  tempera-
   tures  in deeper layers of the photosphere over the faculae tend to be
   lower than the upper photospheric layers.  For these reasons,  faculae
   do  not exhibit limb-darkening when viewed near the solar limbs.  They
   also disappear from view when seen away from  the  solar  limbs  under
   white light, for the same reasons.

        The associated chromospheric plage  can  be  viewed  against  the
   solar  disk  or  near the solar limbs when seen through an appropriate
   monochromatic filter.  By observing the  chromospheric  plage  through
   appropriate  filters,  we have been able to determine the characteris-
   tics of plage associated with active  regions.   For  example,  it  is
   known that plage and/or faculae which form away from active regions do
   not live as long nor do they attain the sizes and intensities found in
   the regions surrounding active sunspot groups.

   _2._5._2.  _P_r_o_m_i_n_e_n_c_e_s _a_n_d _F_i_l_a_m_e_n_t_s

        Prominences are structures seen protruding  from  the  relatively
   cool  chromosphere  into  the  hot  corona.   They typically extend to
   heights of 30,000 or 40,000 km above the chromosphere, but can  attain
   heights  as  high  as  100,000 km in some cases.  Prominences are only
   seen near the solar limbs.
   _________________________
     [3] The magnetic fields associated with the active regions are ulti-
   mately the last detectable remnants to fade away.  The magnetic fields
   therefore, outlive plages and facula.



                               July 14, 1991





                                   - 12 -


        When prominences are viewed against  the  solar  disk,  the  name
   changes to a _f_i_l_a_m_e_n_t.  As prominences rotate into view such that they
   are seen against the solar disk, they  appear  as  long  stringy  dark
   filaments  that  can stretch for distances up to 200,000 km.  Although
   long in appearance, their widths are usually  relatively  small,  near
   about  6000 km.  Prominences and filaments vary considerably in dimen-
   sions.  They can be very small, the size of chromospheric spicules, or
   very large as was mentioned above.

        Prominences form both near active regions and  away  from  active
   regions over apparently quiet areas of the solar surface.  Prominences
   which originate away from centers of activity are generally  known  as
   _q_u_i_e_s_c_e_n_t  _p_r_o_m_i_n_e_n_c_e_s,  and  are  usually less active and live longer
   than prominences which form near active regions.   _A_c_t_i_v_e  _p_r_o_m_i_n_e_n_c_e_s
   are those which form near active regions.  The fluctuating energy out-
   put and unstable environment cause active prominences to display  some
   impressive  forms  of  activity.   Active  prominences are, as a rule,
   associated with sunspots and occur in the earlier part of the life  of
   a  center  of activity.  This does not mean that quiescent prominences
   cannot undergo sudden changes.  For  example,  sometimes  a  quiescent
   prominence  starts to rise slowly, but rises faster in the middle than
   at the ends, thus developing into an arch.  As the arch expands at  an
   increasingly  higher  velocity,  attaining several hundred km/sec, the
   material disperses and fades to  invisibility.   Such  _e_r_u_p_t_i_v_e  _p_r_o_m_-
   _i_n_e_n_c_e_s  have  been  known  to reach heights of 1.5 million kilometers
   above the solar limb.  When seen on the disk  as  filaments,  eruptive
   prominences  are  represented by the sudden disappearance of the fila-
   ment (or a _d_i_s_a_p_p_e_a_r_i_n_g _f_i_l_a_m_e_n_t).  Disappearing filaments (and  thus,
   eruptive  prominences) can release huge quantities of energy which can
   produce terrestrial impacts here on the earth.

        Erupting prominences and disappearing filaments are one  and  the
   same,  only  viewed  at  different  positions  on the solar disk.  The
   majority of eruptions or disappearances are only temporary.   Usually,
   the original prominence reforms over the same region and in nearly the
   same configuration within a few days.

        There are many different types  of  prominences  associated  with
   varying levels of solar activity.  Prominences of greatest interest to
   us are _s_u_r_g_e-_t_y_p_e and _l_o_o_p-_t_y_p_e prominences, which are  manifestations
   of unusually energetic solar activity.  Flares often produce surge and
   loop type prominences.

        The typical surge-type prominence is a confined jet  of  material
   rising  out  of  the  chromosphere  with a velocity of several hundred
   km/sec to a height of some tens of  thousands  of  kilometers.   After
   reaching a maximum height, the material usually falls back to the sur-
   face along nearly the same path as the  outgoing  matter.   Like  most
   prominences,  surges  show  fine  structures in the form of threads of
   luminous matter.  Several surges can occur  in  the  same  region  and
   using  the  same  trajectories as other surges.  The lifetimes of most
   surges are short, usually lasting only a few  minutes,  although  they
   have been known to endure for several hours.




                               July 14, 1991





                                   - 13 -


        Loop-type prominences are likewise, associated with  considerable
   amounts  of  flare  and  coronal activity.  The prominence loops often
   form from bright knots or arcs at considerable heights above the limb,
   perhaps  100,000  km.   Material  streams  down  along two main curved
   arteries, and soon the prominence takes on a true loop shape, with the
   two  arms meeting in a single point near, if not in, a sunspot.  Loops
   usually last a few hours or less.  At the end  of  their  lives,  they
   fade and disintegrate.  Quite often, the last visible features are the
   high, now fainter, knots from which they originated.  Loop prominences
   exhibit a peculiar spectral line called the _c_o_r_o_n_a_l _y_e_l_l_o_w _l_i_n_e.  This
   spectral type (made from Ca XV) indicates that the temperature of  the
   medium  surrounding  the  loop  is high.  Moreover, the spectra of the
   loop prominences themselves point to temperatures as  high  as  almost
   any  found  among  prominences  and  bear a close resemblance to flare
   spectra.

        Although there are similarities in activity between quiescent and
   active  prominences,  active prominences are always more energetic and
   have higher temperatures.  Quiescent prominences have kinetic tempera-
   tures  of  around  6,000  to 15,000 degrees Kelvin, while active prom-
   inences may have temperatures that exceed  25,000  to  50,000  degrees
   Kelvin.   Loop  and  surge  type  prominences most often exhibit these
   higher temperatures.

        The average lifetime of a filament is  about  25  days.   Compare
   this with the lifetime of a quiescent prominence, which can last up to
   eight or nine solar rotations.  Quiescent prominences  are  therefore,
   considerably  more  stable unless an active region forms near a quies-
   cent prominence.

        Filaments tend to migrate toward the nearest  heliographic  pole.
   They  form near the sunspot-forming zones and proceed to travel toward
   the poles.  As they  travel,  shorter  filaments  often  combine  with
   longer  filaments  to form a very long filament chain.  Many filaments
   do not manage to make it to the solar poles.  Indeed,  active  regions
   can  completely  annihilate  filaments which wander into their domain.
   Filaments can also simply disintegrate over time.  However,  the  gen-
   eral tendency is for poleward movement of the filaments.

        The high latitude filament zone becomes most prominent during the
   sunspot  minimum  years.  During these years, the polar filament zone,
   known as the _p_o_l_a_r _c_r_o_w_n, continues to move poleward  during  the  new
   solar  cycle.  Polar filaments are characteristically more stable than
   filaments near the sunspot forming zone (nearer to the solar equator -
   ranging from about 30 degrees latitude during sunspot minimum to about
   5 degrees in latitude during sunspot maximum).

   _2._5._3.  _S_o_l_a_r _F_l_a_r_e_s

        One of the most powerful natural explosions known to man  is  the
   solar  flare.   This relatively short-lived explosion occurs over com-
   plex sunspot groups.  They can be immensely powerful.  A  large  solar
   flare can release energy equivalent to a 10 billion megaton bomb.




                               July 14, 1991





                                   - 14 -


        Solar flares can be devastating to our  terrestrial  environment.
   Among  some  of  the  effects  which are experienced in and around the
   earth are bombardments of huge doses of ultraviolet  radiation,  which
   have  been  linked  to  global  reductions in the ozone concentrations
   which protect us from hard ultraviolet radiation.  Flares can send out
   vast  quantities  of  highly energetic protons which can penetrate our
   Earths atmosphere to tropospheric heights.  Some powerful flares  have
   been  well  correlated  with  anomalies  in  atmospheric  circulation,
   affecting our weather and climate  for  relatively  short  periods  of
   time.   Flares  have  completely knocked out radio communications over
   long distances and have caused significant disruptions  in  ground-to-
   satellite  and satellite-to-ground communications.  The massive inter-
   planetary shockwaves which can propagate outward from  powerful  solar
   flares  can  create  exceedingly  intense geomagnetic storms which can
   cause a multitude of problems, such as a  lack  of  compass  accuracy,
   loss  of  radio  communications,  and heavy currents induced into long
   conductive elements such as pipelines, railway tracks,  telecommunica-
   tions cables, and electrical power transmission lines.  Strong geomag-
   netic storms have caused electrical  power  transformers  to  explode,
   large-scale blackouts for millions of people, and a great many electr-
   ical brownouts and surges.  The shockwaves from solar  flares  (sudden
   changes  in the velocity, density and pressure of the solar wind) have
   caused satellites to  begin  tumbling  out  of  control.   The  highly
   charged particles which engulf the environment of satellites have also
   damaged the electronic components in some satellites.   Indeed,  solar
   flares can have a profound influence on our terrestrial environment.

        Solar flares may be defined as a sudden release of energy causing
   a  sudden  brightening  of  the chromosphere.  It is important to note
   that flares do _n_o_t occur at the surface of the photosphere  (the  area
   that  we  discern  as the surface with our eyes).  Flares are _c_h_r_o_m_o_s_-
   _p_h_e_r_i_c phenomena, and as such, occur above the photospheric regions.

        The energy released by solar flares comes  from  magnetic  energy
   which  has  been stored and accumulated over time in an active region.
   Generally, solar flares require strong magnetic  gradients.   This  is
   particularly  true for the more powerful class of flares known as _p_r_o_-
   _t_o_n _f_l_a_r_e_s.

        The process whereby flares occur is  basically  as  follows.   An
   active  region  forms  and  develops.   As  it developes, the magnetic
   fields associated with the sunspot group intensify.  Gradients between
   opposite  poles  of  the  magnetic  fields  associated with the active
   region increase.  This process may be represented by an  elastic  that
   is stretched over time to near the breaking point.  At some point, the
   elastic suddenly snaps, releasing all of its stored energy in  a  very
   short time.  The sudden release of energy that was pent up in the mag-
   netic fields causes a sudden and intensive explosion which  superheats
   the  chromosphere and nearby regions to temperatures of near 5 million
   degrees Kelvin.  Particles are often explosively ejected from the  sun
   at  this  time,  being accelerated to near relativisitic speeds within
   fractions of a second. These types  of  flares  are  known  as  _p_r_o_t_o_n
   _f_l_a_r_e_s and can have a strong influence on our terrestrial environment.
   The  extremely  high  temperatures  emit  high  doses  of  x-ray   and



                               July 14, 1991





                                   - 15 -


   ultraviolet  radiation.   Within  eight  minutes, the x-ray and ultra-
   violet radiation reaches the earth, causing instantaneous  and  abnor-
   mally  high levels of ionization in the ionosphere, which consequently
   affects radio communications.   Within  about  an  hour,  the  highly-
   accellerated high-energy solar protons traverse the vast distance from
   the sun and slam into the earth.  Many of  the  high-energy  particles
   are redirected by the Earths magnetic field to the polar regions where
   they may penetrate to ground levels and cause a _p_o_l_a_r  _c_a_p  _a_b_s_o_r_p_t_i_o_n
   _e_v_e_n_t  (or  _P_C_A).   The  unusually  high  proton  density of the space
   environment at satellite altitudes are called _s_a_t_e_l_l_i_t_e _p_r_o_t_o_n  _e_v_e_n_t_s
   and  are  responsible  for causing satellite communication disruptions
   and potential damage to satellite systems.

        The massive explosions from flares  may  last  from  only  a  few
   minutes to many hours.  The huge conservatively rated class X-15 flare
   of March 6, 1989 maintained its explosive power for  ten  hours,  com-
   pared to the more typical 30 minutes for flares.  It was an exception-
   ally powerful flare, perhaps the most powerful flare ever recorded.

        Flares are not usually visible in white light.  That is, we can't
   normally  see  flares  with  our  naked  eyes.   The majority of light
   released by major flares occur  in  a  region  of  the  spectrum  that
   requires  a  monochromatic light filter (such as an H-alpha filter) to
   be seen.  Only in rare cases, during particularly intense flares,  can
   they  be  seen in white light.  These cases are reserved for the rogue
   flares, which superheat the photosphere and cause simultaneous bright-
   enings  of  the  photosphere.  These brightenings can be seen in white
   light, but last only momentarily.  Flares are therefore,  not  usually
   seen  in white light since most flares do not attain the high tempera-
   tures necessary to superheat the photosphere to  levels  that  can  be
   detected in white light.

        It typically requires approximately 36 to 48 hours for a powerful
   flare  to  produce significant geophysical events.  By calculating the
   time it takes for flare-related impacts to affect the earth, the velo-
   city  of  the travelling solar material can be calculated.  Generally,
   the higher the velocity of the material, the more severe  the  terres-
   trial  impacts tend to be.  Flares which eject matter at speeds suffi-
   cient to cross the sun-earth boundary in 24 hours are capable of  pro-
   ducing profound terrestrial effects.  However, particle velocities are
   not the only aspects which must be  considered.   Interplanetary  mag-
   netic fields and plasma densities are also important factors, but will
   not be discussed here in any great detail.  Suffice  it  to  say  that
   plasma  densities  (that  is,  the  density  of  the cloud of material
   ejected by flares) that are relatively high  tend  to  produce  strong
   effects  at the Earth.  Likewise, the magnetic fields contained in the
   cloud of particles ejected  by  flares  have  effects  on  geomagnetic
   activity.  Interactions between the Earths magnetic field and the mag-
   netic fields in the cloud of particles can cause field lines  to  cou-
   ple,  link and destroy each other.  This process releases vast quanti-
   ties of energy and heat into the Earths atmosphere which  causes  both
   auroral  activity  and  intense magnetic storms.  More will be said on
   this in later sections.




                               July 14, 1991





                                   - 16 -


   _2._5._4.  _P_o_l_a_r _C_a_p _A_b_s_o_r_p_t_i_o_n _E_v_e_n_t_s

        Perhaps one of the most astonishing  influences  of  large  solar
   flares  are  the polar cap absorption events (also known as PCA events
   or PCAs).  PCAs occur shortly after the eruption of a powerful  proton
   flare.   The  proton flare ejects large quantities of solar protons at
   high velocities towards the earth.  Within a few  hours,  these  high-
   energy  particles  arrive  at  the  earth.   Since the particles which
   arrive at the earth have an electrical charge, they are influenced  by
   the  magnetic  field  of  the earth.  The Earths magnetic field effec-
   tively steers the high-energy protons to the north and  south  geomag-
   netic  poles.   Here,  the  particles slam into the ionosphere at very
   high speeds.  Their energy permits them to penetrate to deep levels in
   the  Earths  atmosphere.  As they penetrate, they collide with consti-
   tuents of the Earths atmosphere.  When they do  so,  they  ionize  it.
   This  ionization prevents radio signals from being reflected by normal
   ionospheric refraction.  Hence, long distance radio communications are
   severely inhibited during PCA events over the polar regions.

        The intense ionization which occurs during strong PCA events  are
   usually  confined  to  the  polar  regions.   However, the latitudinal
   dependence of PCA-related ionization  is  strongly  dependent  on  the
   intensity  of  the  event.   Particularly intense PCAs may cause radio
   blackouts for regions  down  to  geographical  latitudes  of  near  50
   degrees.   Thus,  middle  latitude regions may also be affected by PCA
   events.

        The intensity of PCA events is measured at polar  stations  using
   instruments  called  _R_i_o_m_e_t_e_r_s  (Relative Ionospheric Opacity meters).
   These basically measure the transparency  of  the  Earths  ionosphere.
   During  PCA events, absorption of extra-terrestrial radio signals (ex.
   cosmic noise) is enhanced and the  corresponding  decrease  in  signal
   intensities  is  recorded  by  this  instrument. A PCA occurs when the
   absorption detected by the riometer exceeds  2.0  dB  during  daylight
   hours  or  0.5 dB during the night.  PCAs usually reach a peak absorp-
   tion level soon after the flare and may require several days  (perhaps
   up to several weeks) to return to preflare levels.

        PCAs also produce _g_r_o_u_n_d _l_e_v_e_l _e_v_e_n_t_s (_G_L_E), where the  penetrat-
   ing  solar  particles  actually reach ground levels briefly over polar
   regions.  These events are detected using instruments  called  _n_e_u_t_r_o_n
   _m_o_n_i_t_o_r_s.   When  the  neutron  monitor  trace increases by 5% or more
   above normal background levels, a ground level event is said to be  in
   progress.

        Associated with GLEs are phenomena called _F_o_r_b_u_s_h _D_e_c_r_e_a_s_e _E_v_e_n_t_s
   (or  _F_o_r_b_u_s_h  _D_e_c_r_e_a_s_e_s).   These  events are also measured by neutron
   monitors and are defined when the neutron monitor trace  decreases  5%
   or  more  below  normal background levels.  Forbush decreases and GLEs
   are usually associated with large  geomagnetic  storms  (discussed  in
   later sections).






                               July 14, 1991





                                   - 17 -


   _2._5._5.  _S_i_g_n_i_f_i_c_a_n_c_e _o_f _S_w_e_e_p _F_r_e_q_u_e_n_c_y _E_v_e_n_t_s

        It has been known for years that the sun emits radio waves over a
   wide  range  of  frequencies.  Although solar radio astronomy began in
   1942, it never really became a serious area of  research  until  after
   the  second  world  war  in 1945 and 1946.  The years of research have
   yielded some interesting results, some of which  we  will  examine  in
   this section.

        The sun radiates three types of radio emission.  (1) The constant
   background  continuum  of the quiet sun, observed throughout the radio
   spectrum, caused by thermal emission in the chromosphere  and  corona.
   (2) The slowly varying component, most readily observed at wavelengths
   of 3 to 60 cm.  This component is associated with sunspots and plages.
   (3)  The transient enhanced radiations, including noise storms and the
   several types of burst radiations.

        The radio burst radiations which we will concentrate  on  in  the
   following sections have specific characteristics that allow them to be
   separated into groups or types.  We  will  concentrate  on  the  radio
   emissions identified as _T_y_p_e _I_I and _T_y_p_e _I_V sweep frequency events.

        The term _s_w_e_e_p _f_r_e_q_u_e_n_c_y is used to describe the behavior of  the
   radio  emissions  as  observed  at  earth.  These emissions consist of
   intensified regions of the radio spectrum which drift (or sweep)  from
   higher  to  lower  frequencies.   For example, during a major flare, a
   Type II radio sweep means that during the flare,  part  of  the  radio
   spectrum  observed  intensified  (ie.   the  noise  became louder) and
   drifted from high frequencies down to lower frequencies.  This is what
   is meant by a _s_w_e_e_p _f_r_e_q_u_e_n_c_y _e_v_e_n_t.

        There are basically five major types of radio emissions which are
   commonly   categorized.   These  types  are  categorized  using  roman
   numerals and depict different aspects and phenomena  occuring  on  the
   sun  at  radio wavelengths.  The following sections very briefly cover
   the slowly varying component, as well as emissions of types I, III and
   V.   A  more extensive analysis of the slow drift bursts (types II and
   IV) will follow, as they pertain  more  to  the  occurrence  of  major
   geomagnetic storms than the other types.

   _2._5._5._1.  _T_h_e _S_l_o_w_l_y _V_a_r_y_i_n_g _C_o_m_p_o_n_e_n_t

        Radio frequency radiation  from  the  sun  has  a  characteristic
   minimum  base-level which is generated by the thermal processes occur-
   ring in the sun.  Over a period of days, this base-level  radio  emis-
   sion  can  be  observed  to  increase or decrease in intensity.  These
   changes comprise the slowly varying component.  In 1959,  Covington[4]
   showed  that  the  monthly average of the emission intensity varies in
   phase with the solar cycle.  In fact, it was observed that the  slowly
   varying  component  is  closely  associated  with sunspots and plages.
   _________________________
     [4] (1959) Solar emission at 10 cm wavelength,  Paris  Symposium  on
   Radio Astronomy, Stanford University Press, page 159.



                               July 14, 1991





                                   - 18 -


   Shortly thereafter, it was discovered that for the  20  cm  radiation,
   the  maxima of the solar radio emission represented the area overlying
   the brightest plage areas rather than sunspots.

        Since the slowly varying component occurs at wavelengths  ranging
   from 3 cm to over 100 cm, the range in height of the originating emis-
   sion above the  chromosphere  can  be  considerable,  from  10,000  to
   300,000  km.   At  longer  wavelengths,  the  slowly varying component
   begins to interact with radio bursts which  originate  higher  in  the
   corona.   The  greatest  effect  of  the  slowly  varying component is
   observed over the frequencies of 7 to 60 cm.

        The slowly varying component is  not  particularly  important  in
   determining  potential terrestrial impacts such as geomagnetic storms.
   They are, however important in determining the potential activity  and
   intensity  of  specific active regions (or of the entire visible solar
   disk as a whole).  The  solar  flux  (at  a  wavelength  of  10.7  cm)
   represents  the  slowly varying component and is very useful in deter-
   mining the activity of the sun as a whole.

   _2._5._5._2.  _T_y_p_e _I _B_u_r_s_t_s _a_n_d _R_a_d_i_o _N_o_i_s_e _S_t_o_r_m_s

        Radio noise storms are violent  increases  in  the  intensity  of
   noise  originating  from solar coronal regions.  Noise storms are gen-
   erally comprised of many (hundreds to thousands) of discrete bursts of
   noise, which have been identified and named as _T_y_p_e _I _b_u_r_s_t_s, or _s_t_o_r_m
   _b_u_r_s_t_s.

        Radio noise storms and Type I  bursts  are  associated  with  the
   intense  magnetic  fields  in  active  regions,  which rise to coronal
   heights and interact with the corona to produce the noise.

        Solar flares generally do not affect the frequency of  occurrence
   of  Type  I  bursts.   They appear to be somewhat independent of flare
   phenomena and are correlated more with the magnetic fields  in  active
   regions than with flares.

        These types of burst radiations are not of particular  importance
   to  those  interested  in  predicting  terrestrial  impacts.  For more
   information on these types of radio emissions, consult the many  books
   available  at  your  public or University library regarding flares and
   solar radio emissions.

   _2._5._5._3.  _T_y_p_e _I_I_I _R_a_d_i_o _B_u_r_s_t_s

        Type III radio emissions occur almost daily during the solar max-
   imum years.  Both Type III burst and Type V bursts are associated with
   _f_a_s_t _d_r_i_f_t events.  Fast drift events are those where the frequency of
   the  radio  emission is observed to drift rapidly from higher frequen-
   cies to lower frequencies.

        It has been determined that the drift rate is  dependent  on  the
   frequency  being  observed.  For example, the drift rate at 200 MHz is
   about 150 MHz per second, while the drift rate  at  lower  frequencies



                               July 14, 1991





                                   - 19 -


   such as 25 MHz is lower, near about 4 MHz per second.

        These sweep frequency events are  caused  by  outward-propagating
   waves  which  travel at high velocities ranging from 20% to 80% of the
   speed of light.  The duration of most Type  III  bursts  is  about  30
   seconds  in  the  low frequency range, but varies with increasing fre-
   quency.  Burst durations on the higher frequencies vary from 3  to  10
   seconds at 100 MHz to less than 1 second above about 500 MHz.

        Type III bursts tend to occur in groups, ranging  from  a  single
   burst  to  as many as 100 grouped together.  As the number of closely-
   spaced bursts increases, the intensity of the observed emission  like-
   wise increases.

        It has been determined that about 50 to 60 percent  of  Type  III
   bursts  occur  within  10 minutes of the start of a flare or subflare.
   The greater the number of bursts in a group or the greater the  inten-
   sity  of  a  burst,  the  more  probable  the  association with flares
   becomes.

        Aside from the facts already stated, Type III bursts do not  have
   any  significant  terrestrial  impacts.   They can enhance atmospheric
   ionization, but cannot produce geomagnetic storms.

   _2._5._5._4.  _T_y_p_e _V _R_a_d_i_o _B_u_r_s_t _E_m_i_s_s_i_o_n_s

        Type V radio bursts tend to follow Type III radio  bursts.   This
   type  of  radiation  consists  of a wide-band emission of considerable
   intensity, particularly at the lower frequencies near  100  MHz,  with
   durations  from  30  seconds  to 5 minutes.  Type V bursts are usually
   confined to the lower frequencies, and have been observed from near 25
   MHz  to frequencies in excess of 150 MHz.  However, most of the radia-
   tion remains confined to frequencies near 100 MHz.

        Type V burst velocities average about 3000 km/second.   They  are
   very  highly  correlated  with solar flares.  Between approximately 60
   and 90 percent of all Type V radio bursts occur within about 5 minutes
   of the start of a flare or subflare.  They are more closely correlated
   with subflares than flares of greater importance, but  are  also  fre-
   quently observed to occur in conjunction with flares of greater impor-
   tance.

        These radio emissions are not related  to  geophysical  phenomena
   produced  by large flares.  There is no real correlation between these
   types of radio bursts and significant terrestrial impacts.

   _2._5._5._5.  _T_y_p_e _I_I _R_a_d_i_o _B_u_r_s_t_s

        Type II radio bursts represent slow-drift sweep frequency events.
   That is, the frequencies of the radio emissions decrease rather slowly
   when compared to the drift rates for Type III radio bursts.   Type  II
   radio  bursts  are  important  to  solar terrestrial physicists, since
   their occurrence can increase the risks for terrestrial impacts,  par-
   ticularly if associated with Type IV burst emissions (discussed in the



                               July 14, 1991





                                   - 20 -


   next section).

        Almost all Type II events are coincident  with  flares,  although
   most  flares  do  not produce Type II bursts.  In fact, Type II bursts
   occur rather rarely and are generally only associated with  flares  of
   greater importance (ex. major flares).

        These bursts consist of emission in narrow frequency  bands  that
   slowly  drift from high to low frequencies.  The average drift rate is
   about 300 KHz per second at 100 MHz.  As a  rule,  the  bandwidths  of
   Type II bursts are quite narrow, sometimes only a few MHz in the lower
   frequencies near 100 MHz.  Most slow-drift bursts of  this  type  fade
   before  reaching frequencies near 25 MHz, although Type II bursts have
   been known to drift down to frequencies below 25 MHz.

        The drift of a burst from higher  to  lower  frequencies  may  be
   interpreted  as a result of the motion of the burst source through the
   corona.  Methods have been adopted to calculate the approximate  velo-
   cities of the burst sources through the corona.  The methods relied on
   most heavily bring the average burst velocities to  between  1000  and
   1500  km  per second.  These values may be in error by a small amount,
   since the density of the coronal region where the  burst  source  ori-
   ginated  from  must be used in the calculations and this value must be
   approximated from models of the corona, not from actual measurements.

        Type II bursts are often associated with the expulsion  of  solar
   material  into  interplanetary  space.  By calculating the approximate
   velocity of the material using the method mentioned above, the approx-
   imate  intensity of terrestrial impacts can be roughly determined.  If
   the Type II burst is clearly associated with a well-positioned  flare,
   the  probability for increased geomagnetic activity increases dramati-
   cally.  Moreover, it has been found that magnetic  activity  tends  to
   increase  between  1.5  and  2.5  days after the occurrence of Type II
   bursts.  This correlates well  with  ejected  material  travelling  at
   speeds near 1000 km/second.

   _2._5._5._6.  _C_o_n_t_i_n_u_u_m _T_y_p_e _I_V _R_a_d_i_o _E_m_i_s_s_i_o_n_s

        Type IV radio emissions often follow the slow-drift Type II radio
   bursts.  Type IV emissions are primarily stable emissions which do not
   drift in frequency very  much  (if  at  all).   They  have  very  wide
   bandwidths,  sometimes more than eight octaves and often lie at higher
   frequencies than those occupied by most radio noise  storms  (see  the
   section  on  Type I bursts).  The greatest intensity of this radiation
   occurs at frequencies below 250 MHz.  Often, Type IV  emissions  occur
   simultaneously  at  high  and low frequencies in two separate areas of
   the spectrum.  Type IV bursts frequently occur in  the  low  frequency
   areas  between  7  MHz  and 38 MHz and very often follow Type II slow-
   drift bursts.

        A high percentage of Type IV bursts coincide  with  solar  flares
   and  burst  emissions  of Type II.  Generally, Type IV bursts occur in
   conjunction with more powerful solar flares, which is also  in  agree-
   ment with the behavior of the Type II bursts which they often follow.



                               July 14, 1991





                                   - 21 -


        Further evidence of their association with major  flares  is  the
   confirmed  association  with  the  occurrence  of polar cap absorption
   events, where high energy solar  protons  penetrate  into  the  Earths
   atmosphere.   They  are  therefore,  also associated with the powerful
   proton flares and often accompany the expulsion  of  high-speed  solar
   protons into interplanetary space.

        The correlation between magnetic storms and  Type  IV  events  is
   exceedingly  high  when  Type  IV events are preceded by Type II radio
   bursts.  In most cases, a Type II radio burst followed by  a  Type  IV
   radio  burst  indicate the mass ejection of solar material into inter-
   planetary space.  This material most often causes  geomagnetic  storms
   within 48 hours of the observed event.  Moreover, the association of a
   Type II followed by a Type IV radio burst is  very  highly  correlated
   with the occurrence of major solar flares.

        This is extremely helpful to the person interested in  predicting
   potential  terrestrial impacts caused by major flares.  By calculating
   the velocity of the Type II burst and noting the intensity of both the
   Type II burst and the accompanying Type IV burst, the potential sever-
   ity of terrestrial effects can be predicted  with  moderate  accuracy.
   Given  the  typical  lag  time  between flare occurrences and magnetic
   storms, the  forecaster  can  generally  foretell  the  occurrence  of
   increased  magnetic  activity  (and  therefore  radio  propagation and
   ionospheric conditions) to within a 2 to 3 day period.



   _3.  _T_h_e _E_a_r_t_h_s _M_a_g_n_e_t_i_c _F_i_e_l_d

        All of the objects in our solar system have magnetic fields.  The
   earth  is no exception.  We have used our magnetic field for centuries
   as a reliable tool in navigation.  Little did we realize back then how
   vital  our  magnetic  field  is.   Without a magnetic field, the earth
   would be subject to harmful radiations from the  sun.   Life  probably
   would not exist as it does today.

        The Earths magnetic field has two poles.  As any boy-scout knows,
   a  compass  points  toward the north and south geomagnetic poles.  But
   there is a third component of the magnetic field that most people  are
   unaware  of.   This  is a vertical component.  Not only does a compass
   needle point north and south, but it also tilts at  an  angle  to  the
   horizontal plane.  As one moves closer toward the magnetic poles, this
   "dip angle" increases towards the vertical.  At the magnetic poles,  a
   compass  needle  would  point  straight up and down and the horizontal
   movement (ex. the movement pointing north or  south)  would  be  unde-
   fined.

        If the magnetic field of the earth were  drawn  schematically  on
   paper,  it  would  resemble a spherical shell with lines of force pro-
   pagating outward from the poles and  connecting  over  the  equatorial
   regions.   This shape is characteristic for a spherical dipole magnet.
   A dipole field is a good approximation of the shape of the Earths mag-
   netic  field.   However  it  is  not  a  perfect representation.  Some



                               July 14, 1991





                                   - 22 -


   anomalies from the perfect dipole exist.   But  for  our  purposes,  a
   spherical  dipole field will suffice in describing the phenomena which
   occur.

        The solar wind has a profound  influence  on  the  shape  of  the
   Earths  magnetic  field.  The solar wind is analagous to winds that we
   experience here on earth, except that the winds  are  created  by  the
   outflow  of  energy  from  the sun.  Just like the Earths winds, solar
   winds can gust and fluctuate in speed.  Solar flares can cause extreme
   gusts  in  both  speed  and pressure which can affect the stability of
   objects (such as satellites) in space.

        The pressure from the solar wind transforms our  Earths  magnetic
   field  into  a  comet-like appearance.  The "head" of the "comet" sur-
   rounds the earth and the "tail" extends outward away  from  the  earth
   for millions of miles (well beyond the orbit of the moon).  The region
   near the head of the magnetic field where the solar wind  first  makes
   contact with the Earths field is called the _b_o_w _s_h_o_c_k region.  This is
   a transition zone where particles of the  supersonic  solar  wind  are
   abruptly  slowed  to subsonic speeds.  Particles and radiations can be
   deflected around the earth by this region.  It therefore serves  as  a
   type of "shield", protecting us against certain harmful radiations.

        The Earths magnetic field is flexible, like a bed of springs.  It
   reacts  to  increased  solar  wind  pressure by compressing inward and
   reacts to decreased wind pressure by expanding outward.  Strong  solar
   wind gusts created by powerful solar flares are capable of compressing
   the Earths magnetic  field  to  altitudes  near  where  geosynchronous
   satellites  reside.   Compressions of this magnitude generate enormous
   currents in the Earths magnetosphere  which  in  turn  spawn  powerful
   geomagnetic  storms.   These  storms  are closely monitored around the
   world.  Moreover, fluctuations in the speed and density of  the  solar
   wind are also responsible for producing geomagnetic storms.

        In the following sections, we  will  discuss  the  properties  of
   geomagnetic  storms,  substorms, accompanying auroral activity and the
   combined effects on radio propagation.

   _3._1.  _G_e_o_m_a_g_n_e_t_i_c _S_u_b_s_t_o_r_m_s

        When the conditions and characteristics of the solar wind  change
   or  fluctuate  rapidly,  the  geomagnetic  field can become disturbed.
   Instabilities also result when interactions with solar magnetic fields
   occur.   Instabilities  in  the  geomagnetic field often result in the
   generation of electrical currents in the magnetosphere and  ionosphere
   which  in  turn, produce accompanying geomagnetic fluctuations detect-
   able at ground level.

        Geomagnetic substorms are relatively  short-lived,  lasting  any-
   where  from  less  than  30 minutes to as much as several hours.  Sub-
   storms are most prevalent in polar and auroral-zone  latitudes  (lati-
   tudes  above about 55 to 60 degrees geographic latitude - although the
   zones are more a function  of  geomagnetic  latitude  than  geographic
   latitude).



                               July 14, 1991





                                   - 23 -


   _3._2.  _G_e_o_m_a_g_n_e_t_i_c _S_t_o_r_m_s

        When many substorms occur over a period of  a  day  or  two,  the
   entire  event  as  is called a geomagnetic storm.  Intense geomagnetic
   storms may last many days, but most occur over a period of  24  to  48
   hours.

        Geomagnetic storms undergo three  basic  stages  of  development.
   These stages are outlined as follows.

        First, a shock wave from the sun slams into the earth.  This sud-
   den  gust  and  pressure  change in the solar wind produces a magnetic
   impulse that is detected all around the world in a matter of  minutes.
   This  magnetic  impulse is called a _s_u_d_d_e_n _s_t_o_r_m _c_o_m_m_e_n_c_e_m_e_n_t (_S_S_C) or
   _s_u_d_d_e_n _c_o_m_m_e_n_c_e_m_e_n_t (_S_C).  This marks the initial phase of a  magnetic
   storms' development.

        During the SSC, the intensity of the horizontal component of  the
   Earths  magnetic  field increases.  This increased intensity is due to
   the sudden compression of the Earths magnetosphere.  During  the  next
   several  hours,  the  magnetic  field  remains fairly steady with only
   minor fluctuations.

        Approximately three to six hours after the SSC, the second  phase
   or  _m_a_i_n _p_h_a_s_e of the storm begins.  At this time, the Earths magnetic
   field begins to fluctuate wildly.  The main phase coincides  with  the
   arrival  of  the  main  cloud of particles that are ejected from major
   flares or coronal holes.  It may take several days for  the  earth  to
   pass  through  this  cloud  of  solar debris.  During this period, the
   earth can  experience  major  magnetic  fluctuations  (substorms)  all
   around the world.

        Following the main phase is the recovery phase.  This  phase  may
   drag  on  for  days following the main phase.  It is a period when the
   geomagnetic field begins to return to normal.  When the extreme condi-
   tions  in  space abate, the magnetosphere begins to relax.  This phase
   of recovery may still experience some periods  of  substorm  activity,
   but  overall  activity  noticably decreases.  Within several days, the
   geomagnetic field returns to normal and substorming ceases.

        Not all geomagnetic storms follow these stages  precisely.   Many
   storms,  for  example,  do not begin with a sudden commencement.  Many
   storms simply enter the main phase gradually.  These types  of  storms
   are  called  _g_r_a_d_u_a_l  _c_o_m_m_e_n_c_e_m_e_n_t storms and are generally associated
   more with coronal holes than with energetic solar flares.   They  also
   tend to be less intense than storms which are associated with SSCs.

        Occasionally, a sudden and short-lived  shock  impacts  with  the
   Earths  magnetic field.  This _s_u_d_d_e_n _i_m_p_u_l_s_e (or _S_I) is usually a pre-
   cursor to increased geomagnetic activity, although storms preceded  by
   sudden  impulses are usually only of minor intensity (there are excep-
   tions to this, however).  Sudden impulses occur as very-short-duration
   (around  four  minutes)  pulses of increased magnetic intensity.  They
   are easily  seen  on  magnetometer  traces  as  distinct  short-lived,



                               July 14, 1991





                                   - 24 -


   relatively high amplitude "bumps".

   _3._3.  _I_o_n_o_s_p_h_e_r_i_c _E_f_f_e_c_t_s _o_f _G_e_o_m_a_g_n_e_t_i_c _S_t_o_r_m_s

        Geomagnetic storms can have a profound effect on  the  conditions
   in   the  ionosphere,  particularly  over  the  auroral-zone  regions.
   Geomagnetic storms are usually caused by terrestrial interactions with
   solar-ejected  clouds  of  particles.   These  clouds of particles are
   directed by the magnetic field toward the polar regions, but  tend  to
   congregate  along an oval-shaped region known as the _a_u_r_o_r_a_l _z_o_n_e.  In
   this region, where the particle  penetration  is  often  the  highest,
   ionospheric properties and characteristics fluctuate most rapidly.

        Besides being the  primary  particle  penetration  boundary,  the
   auroral  zone  is also associated with the strongest magnetic fluctua-
   tions and levels of instability in the world.  These zones (one in the
   northern  and one in the southern hemisphere), are located at approxi-
   mately 67 degrees geomagnetic latitude (which corresponds somewhat  to
   geographical  latitudes  between  roughly  55  and 70 degrees).  These
   zones often migrate equatorward  as  geomagnetic  activity  increases.
   Hence, for many middle-latitude locations, strong geomagnetic activity
   can place them directly in the heart of the auroral zone  due  to  the
   migration of the auroral zone.

        The ionospheric properties  over  the  auroral  zone  can  change
   rapidly.  This zone is the home of the _a_u_r_o_r_a_l _e_l_e_c_t_r_o_j_e_t, which is an
   oval-shaped core of intense electrical current which  courses  through
   the  ionospheric and magnetospheric regions.  This current (along with
   particle precipitation)  can  cause  large  temperature  anomalies  at
   ionospheric  heights.  The magnetic field and ionospheric densities at
   ionospheric  heights  sensitive  to  radio  communications   likewise,
   undergo  large fluctuations and other anomalies which can affect radio
   propagation conditions.

        The ionosphere basically consists of  four  distinct  regions  of
   ionization.   These  regions,  called layers, are defined according to
   height.  The lowest layer, called the D-region, resides at a height of
   about  70  to  90  km  and appears only during the daylight hours when
   solar radiation is sufficiently high to ionize the ionosphere at these
   relatively  low  heights.   The  E-layer  lies above the D-layer at an
   altitude between 90 and 150 km.  This region, like the D-layer is pri-
   marily  ionized during the day, but can remain ionized sufficiently to
   provide distant radio communications  into  the  evening  hours.   The
   region  above  the E-layer is the F1 layer, which is located at a dis-
   tance of between 150 and 250 km.  During the day, this region is  dis-
   tinct  and  separate  from  the  last  layer of the ionosphere, the F2
   region, which resides at an altitude which varies  between  about  250
   and  400  km.  During the late afternoon and evening hours, the F1 and
   F2 regions merge into a single region of ionization simply called  the
   F-region.

        Auroral activity and maximum  energy  deposition  occurs  in  the
   auroral  zone at a height of about 110 km.  This coincides with the E-
   region of the ionosphere.  During geomagnetic storms,  the  amount  of



                               July 14, 1991





                                   - 25 -


   ionization  in  the  ionosphere over the auroral zone is often intense
   enough to absorb all radio signals which  pass  through  that  region.
   These  _p_o_l_a_r  _b_l_a_c_k_o_u_t_s are usually confined to the high latitudes and
   polar regions, but can  slip  southward  with  the  expansion  of  the
   auroral zone equatorward.

        One of the most pronounced effects  of  geomagnetic  activity  on
   ionospheric  properties  is  the  ability  for VHF radio signals to be
   "bounced" from regions of visual  auroral  activity.   During  intense
   geomagnetic  storms,  the dip angle of the Earths geomagnetic field at
   ionospheric heights over the  auroral  zone  can  deviate  by  several
   degrees.  The deviation introduces a curvature in the dip-angle of the
   magnetic field which serves as an effective medium  for  bouncing  VHF
   signals.   The  curvature, combined with the high levels of ionization
   near E-layer heights permits these high-frequency signals to be  scat-
   terred  by  the  ionospheric  anomaly.  This process is called _a_u_r_o_r_a_l
   _b_a_c_k_s_c_a_t_t_e_r_i_n_g and is a primary source for long-distance VHF  communi-
   cations.

        Geomagnetic storms also influence the maximum usable  frequencies
   (MUFs)  of  the  various  ionospheric layers.  The maximum usable fre-
   quency of the F2 region is affected most profoundly during geomagnetic
   storms.   In most cases, the MUF decreases well below normal values at
   F2 layer heights.  These heights happen to be  most  sensitive  to  HF
   long-distance communications. In some rare instances, however, the MUF
   of the F2 region actually increases during magnetic storms.  The  main
   phase  of  geomagnetic  storms  affect the MUF of the E-layer as well.
   Depressions in the MUF of the E-region are most often associated  with
   large geomagnetic storms.

        The ionization which occurs at E-layer heights is also  responsi-
   ble  for  another  type  of  radio  phenomenon  known  as  _s_p_o_r_a_d_i_c _E.
   Sporadic E occurs when cloud-like areas of enhanced ionization form at
   altitudes between about 90 and 150 km.  These so-called "clouds" drift
   with time and are most prevalent during the daylight hours and  during
   periods  of  geomagnetic  storming  over  the  auroral zones.  Intense
   storming often causes abnormal increases in E-layer ionization,  which
   can result in polar blackouts.



   _4.  _R_a_d_i_o _S_i_g_n_a_l _P_r_o_p_a_g_a_t_i_o_n

        Radio science rests on the discoveries of Ampere, Oersted,  Fara-
   day, and Henry, who developed the principles of electric induction and
   electric and magnetic fields surrounding conductors carrying  current.
   A  single unified electromagnetic theory was achieved between 1867 and
   1873 by the Scottish physicist James Clerk Maxwell.  In 1887, Heinrich
   Hertz  discovered  radio waves and showed that they exhibit all of the
   properties of light waves.

        In 1896, Guglielmo Marconi assembled various items  of  equipment
   developed  by  Hertz,  D.E.  Hughes, Edouard Branly, Oliver Lodge, and
   others, and approached interested British party's with a  proposal  to



                               July 14, 1991





                                   - 26 -


   use  the  Hertzian  waves for commercial communications.  In 1897, the
   Wireless Telegraph and Signal Company was formed for this purpose, and
   by  the end of that year, messages had been sent over a distance of 18
   miles.  Between 1897 and 1899, Marconi developed equipment for  tuning
   transmitters and receivers to the same frequency to avoid interference
   between stations and to conserve the  power  of  the  radiated  waves.
   Shortly  thereafter, in 1900, Marconi successfully transmitted a tran-
   satlantic message to anxious ears in North America.

        Luckily, the ionospheric conditions at that time  were  favorable
   for transatlantic communications.  It may have been quite a setback if
   their attempts at transatlantic radio communications had failed due to
   geomagnetic activity or solar flares.

        Since that first  transatlantic  contact,  we  have  signficantly
   expanded our knowledge of ionospheric radio wave propagation.  We have
   formulated models of ionospheric behavior in propagating  radio  waves
   and  have  learned  of  the  types  of  solar phenomena which can have
   impacts on radio propagation.  In this section, we will briefly  exam-
   ine  some  of  the  more  important aspects of radio propagation as it
   deals with VLF, HF and VHF radio waves.

   _4._1.  _P_r_o_p_a_g_a_t_i_o_n _o_f _V_L_F _S_i_g_n_a_l_s

        Very Low Frequencies (VLF) are those frequencies which range from
   below  1  KHz to approximately 150 KHz.  These frequencies are home to
   the navigational beacons which transmit on these very low frequencies.

        Radio signals in the VLF  range  are  affected  differently  than
   those  in  the  HF  range.   VLF signals are generally enhanced during
   solar flare induced _S_I_D_s (_s_u_d_d_e_n  _i_o_n_o_s_p_h_e_r_i_c  _d_i_s_t_u_r_b_a_n_c_e_s).   Signal
   strengths  of  VLF  signals  have  been found to increase, often quite
   dramatically, during solar flares.  They also tend to become  enhanced
   during  the  initial phase of geomagnetic storms, but may later suffer
   strong absorption during the main phase.

        We will restrict our discussion of VLF propagation to the  above,
   due  to  the  inability of most people to use this band of frequencies
   for any useful communications.  The bandwidth of VLF communications is
   insufficient to permit voice communications.  Hence, this band of fre-
   quencies is not heavily used for day-to-day long-distance  radio  com-
   munications  and  is  not of particular importance to us in this docu-
   ment.

   _4._2.  _H_F _S_i_g_n_a_l _P_r_o_p_a_g_a_t_i_o_n

        The most widely used communications frequencies for long-distance
   communications  are  those  which span the frequency range between 1.8
   MHz and 30 MHz.  At these frequencies, the ionosphere  is  capable  of
   bending radio signals back toward the earth.  This makes long-distance
   communications a viable possibility on the HF bands.

        Radio propagation on the HF bands is most dependent upon  geomag-
   netic activity, auroral activity (both of which determine the state of



                               July 14, 1991





                                   - 27 -


   the ionosphere and which are most applicable over  middle  and  higher
   latitude paths), and solar activity.

        Solar activity can severely affect the propagation  of  HF  radio
   waves  through  the ionosphere.  Significant solar flaring can produce
   isolated temporary periods of radio blackout conditions.  The  intense
   levels  of  radiation  which  accompany strong solar flares ionize the
   Earths ionosphere over sunlit portions of the earth and produce strong
   absorption  levels  capable  of  completely  absorbing  radio signals.
   Flare-related radio blackouts do not occur very  frequently,  however,
   and  are limited only to the rare occasions when complex solar regions
   form and spawn flares of unusual severity.

        When a major solar flare produces radio wave absorption over  the
   sunlit  portions  of  the  earth, the phenomena is called a _s_h_o_r_t _w_a_v_e
   _f_a_d_e (_o_r _S_W_F) and typically lasts between 20  and  50  minutes.   Some
   long-duration  events,  however, can severely affect radio propagation
   for extended periods of many hours.  These cases, however, are usually
   reserved for the large rogue flares which occur most frequently during
   the solar maximum years.

        It is important to note that SWFs do not occur over the dark side
   of  the earth.  Although there is some evidence to suggest some subtle
   night-time effects, they are  generally  restricted  to  the  daylight
   hours.

        Strong solar proton flares frequently  produce  accompanying  PCA
   and  satellite  proton  events.   As was mentioned earlier, PCA events
   have powerful effects on polar and high latitude radio communications.
   They  are  perhaps  the  most severe form of radio absorption that can
   occur over these latitudes.  They can last for many days and can cause
   wide-spread  polar  blackouts on all radio frequencies.  Their effects
   are not confined to the polar  and  high  latitude  regions,  however.
   Strong events can migrate equatorward, and can engulf middle latitudes
   as well.  Low latitudes  are  generally  unaffected  directly  by  PCA
   events.   However,  during  periods of PCA activity, low latitudes are
   restricted in the latitudinal range where they  can  make  radio  con-
   tacts.  They may be completely unable to establish contact with others
   at high or middle latitudes.  They will almost certainly be completely
   unable  to  make contacts at polar latitudes.  Likewise, signals which
   graze the PCA zone may be completely absorbed.  HF transmissions  dur-
   ing the daylight hours over low latitudes during PCA activity are gen-
   erally weaker and less reliable.  Higher powers usually do compensate,
   but may not aid in penetrating to long-distances.  Night-time communi-
   cations during PCA activity over low latitudes are usually not heavily
   affected.    Therefore,  there  is  a  noticable  diurnal  pattern  of
   increased absorption over latitudes during periods  of  PCA  activity.
   Basically,  all latitudes are affected by PCA activity to some degree,
   although high latitudes and polar regions  are  by  far  affected  the
   most.

        Geomagnetic storms can be almost as devastating to high  latitude
   and  polar  latitude  radio  transmissions  as PCAs, although they are
   almost always less constant when compared to PCAs.   That  is,  during



                               July 14, 1991





                                   - 28 -


   geomagnetic  storms,  there  will  usually be periods of time where at
   least poor communications is possible.  During PCAs, however, communi-
   cations  is often completely blacked out with very few (if any) oppor-
   tunities for any HF propagation of radio signals.

        During magnetic storms, auroral activity usually abounds  in  the
   high and polar latitude regions.  Middle latitudes can also experience
   significant periods of strong  auroral  activity  which  can  severely
   impact  radio  communications.  During these periods, HF radio signals
   can become so garbled as to be completely unintelligable.  Rapid  fad-
   ing  of  HF  signals  caused  by  auroral  activity  is called _a_u_r_o_r_a_l
   _f_l_u_t_t_e_r.  Rapid fading and strongly erratic signal strengths over much
   of  the HF spectrum can destroy attempts to communicate during auroral
   and geomagnetic storms.

        Low latitudes are again, generally better off than  higher  lati-
   tudes  during  geomagnetic  storms.  They experience less fading, less
   absorption and less flutter.   However,  even  low  latitudes  do  not
   escape  all  of  the  effects of geomagnetic storming.  Over all lati-
   tudes, the MUF of the F2 region decreases (often quite  dramatically).
   Likewise,  the  MUF  of  the E region also often decreases.  Also, the
   lowest usable frequency (LUF) almost always increases during a geomag-
   netic  storm.  The combined effects of decreased MUF and increased LUF
   effectively narrow the usable HF spectrum.  Often, the F layer becomes
   completely  unusable  for HF communications, as has been observed many
   times with ionosonde maps of the ionospheric layers.  The F region may
   completely  disappears  from  such  maps  during some intense magnetic
   storms.  At other times, there  may  exist  spread-F  which  can  also
   strongly  influence radio communications over all latitudes.  Spread F
   is caused by the scattering of radio signals by anomalies  in  the  F-
   layer  region.   Spread F can limit the amount of information that can
   be transmitted long-distances and can also produce high fading  rates,
   limiting the ability for long-distance radio communications.  The use-
   fulness of packet radio communications can be strongly affected by the
   occurrence of spread F.

        Ionospheric conditions during magnetic storms  vary  considerably
   over  small  changes  in latitude and longitude.  These changes modify
   the character of radio signals which propagate  through  the  changing
   layers  of  the  ionosphere.  Radio propagation over long-distances is
   therefore, difficult to accomplish with  any  reliability  or  success
   during magnetic storms.

        Some very long-distance HF propagation has apparently been accom-
   plished  in  the  past during storm periods, but such contacts are not
   very common.  HF radio signals are more likely  to  be  severely  dis-
   torted  and/or  absorbed  by  the  anomalous  ionization  and magnetic
   behavior in aurorae than to be reliably propagated to  long  distances
   via  aurorae.   However,  for the ambitious soul willing to attempt to
   establish auroral-contacts, note that your best chances  are  via  CW.
   Voice  communications  via aurorae are for the most part, very unreli-
   able, very unintelligable and suffer severe distortion and  fading  by
   the time they reach their destination.  As will be seen in the follow-
   ing section, VHF radio propagation via auroral backscatter is  a  more



                               July 14, 1991





                                   - 29 -


   reliable method of using aurorae for communications.

   _4._3.  _L_o_n_g-_D_i_s_t_a_n_c_e _V_H_F _S_i_g_n_a_l _P_r_o_p_a_g_a_t_i_o_n

        Under most normal conditions, long-distance VHF  signal  propaga-
   tion  is next to impossible.  Frequencies of 144 MHz are almost always
   well beyond the  critical  frequencies  for  the  ionospheric  layers.
   Attempts to transmit VHF signals long distances by the same means used
   for HF signals  will  prove  fruitless  in  most  cases.   Frequencies
   transmitted  to  the  ionosphere  simply  pass through it and out into
   space.  Only under special conditions are VHF signals capable of being
   transmitted long-distances via ionospheric properties.

        Probably one of the best known methods  whereby  this  is  accom-
   plished  is  via  sporadic-E.   As was mentioned in previous sections,
   sporadic E occurs when isolated areas  of  enhanced  ionization  drift
   into  the  area.  Radio signals of unusually high frequencies are able
   to be refracted or scattered by these  localized  "ionization  clouds"
   back to the earth from E-region heights.  These clouds are sporadic in
   nature.  Hence any communications accomplished is likewise  only  tem-
   porary.

        There are several other conditions that have yielded fairly  good
   long-distance  VHF  communications.   However,  determining when these
   conditions will occur is almost as difficult as predicting sporadic E.
   Solar flares which produce SIDs often generate the enhanced ionization
   levels required for long-distance VHF communications.   However,  such
   communications  are  only  possible  over  locations  where  SIDs  are
   observed.  SIDs occur only over the sunlit areas of the  Earth.   They
   also  occur with less intensity over higher latitudes where the eleva-
   tion of the sun makes a shallower angle with the horizon than at lower
   latitudes.   Season  therefore,  plays an important role in the inten-
   sity, duration and frequency of SIDs for VHF propagation.   Low  lati-
   tudes  generally have better luck in propagating VHF signals using the
   enhanced ionization produced during SIDs than high latitudes.   Middle
   latitudes  are  also generally good for such types of propagation, but
   effectiveness decreases during the winter months due to the  decreased
   elevation  angle  of the sun.  High latitudes generally do not experi-
   ence significant SID-related propagation possibilities on VHF frequen-
   cies during the winter months.  However, the prospects improve dramat-
   ically during the summer months.

        The only other major form of potential VHF  communications  takes
   place  during auroral and geomagnetic storms.  Propagation via aurorae
   on VHF frequencies is called _a_u_r_o_r_a_l _b_a_c_k_s_c_a_t_t_e_r_i_n_g  if  long-distance
   contacts  are made as a result of the radio signal bouncing off of the
   aurora.  Likewise, _f_o_r_w_a_r_d _s_c_a_t_t_e_r_i_n_g occurs when signals scatter  off
   of  the aurorae in a forward direction toward the polar regions.  Two-
   way auroral communications  on  VHF  frequencies  is  called  _b_i_s_t_a_t_i_c
   _a_u_r_o_r_a_l _b_a_c_k_s_c_a_t_t_e_r _c_o_m_m_u_n_i_c_a_t_i_o_n_s.

        It is important to note that "scattering" does not mean  "refrac-
   tion."   It  means  radio  signals  are  literally  scattered  off  of
   anomalies  in  the  ionosphere  near  regions  of  auroral   activity.



                               July 14, 1991





                                   - 30 -


   Sometimes  signals  are scattered backwards.  Sometimes they are scat-
   tered forwards. In rare cases where auroral geometry  is  just  right,
   VHF signals can be scattered multiple times off of multiple aurorae to
   achieve significant long-distance communications.  However,  in  these
   cases,  the  quality  of  the radio signal decreases dramatically with
   each contact of the scattering source.

        Scattered VHF signals can  be  discerned  by  their  very  gruff,
   motoring  sounds.   These  types of signals are affected by very rapid
   fading which often fade in and out at frequencies as high as  100  Hz.
   These signals are said to be _s_p_u_t_t_e_r_i_n_g or caused by _a_u_r_o_r_a_l _s_p_u_t_t_e_r.

        In order to achieve auroral backscatter  communications,  auroral
   activity  must  be  visible  low in the horizon.  The more intense the
   activity, the higher the probability for achieving long-distance back-
   scatter  communications.   Directional  antennas are a definite asset,
   since most of the power of the transmitter must be directed toward the
   auroral  region.   The auroral region must be at a low elevation angle
   in order to provide the geometry required for backscattering to occur.
   The  distance of transmissions also increases with increasing distance
   to the aurora.  Hence, low  transmission  angles  are  required.   The
   prospects  for  distant  bistatic  auroral  backscatter communications
   increases if CW communication is used.  CW is much  more  intelligable
   when  distorted  by  aurorae than is voice and therefore can be under-
   stood even when severely distorted by auroral activity.

        The probability of achieving auroral  backscatter  communications
   is  a  function of latitude and geomagnetic activity.  Lower latitudes
   do not experience auroral backscatter communications nearly  as  often
   as  northerly  middle  latitudes  and  high  latitudes  where  auroral
   activity is more prevalent.  However, even at these higher  latitudes,
   such communications depends on the extent of magnetic activity.

        It has been found that auroral  backscatter  communications  only
   become  widespread during major geomagnetic storms.  Minor geomagnetic
   storms are capable of  providing  conditions  necessary  for  isolated
   auroral  communications,  but generally the best communications possi-
   bilities occur when geomagnetic conditions reach  major  storm  levels
   (ex. magnetic K indices of 6 or greater).

        Backscatter communications have two well defined  diurnal  peaks.
   The  largest peak typically occurs in the late afternoon/early evening
   hours.  This peak is not quite so dependent on  geomagnetic  activity,
   although  it  does  appear to be somewhat sensitive to it.  The second
   peak occurs near local midnight, which  coincides  with  the  peak  of
   auroral  activity over most locations.  This second peak appears to be
   heavily dependent on geomagnetic activity.  Widespread  backscattering
   has  been  known  to  occur  during this second peak during periods of
   major geomagnetic storming.  During quiet magnetic periods,  the  peak
   is  almost  non-existant,  indicating  only  very  rare  and  isolated
   incidents of backscatter communications.

        From the foregoing, it is clear that long-distance  VHF  propaga-
   tion  is  indeed  possible,  but requires special conditions before DX



                               July 14, 1991





                                   - 31 -


   communication can occur.  The best times  for  DX  are  in  the  late-
   afternoon  and  early evenings.  The next best opportunities come near
   local midnight during minor to major geomagnetic  storms.   Generally,
   the  prospects  for DX increase with geomagnetic activity.  This is in
   sharp contrast to HF communication, which is seriously  eroded  during
   periods of high geomagnetic activity.



   _5.  _C_h_a_r_a_c_t_e_r_i_s_t_i_c_s _o_f _A_u_r_o_r_a_l _A_c_t_i_v_i_t_y

        The Northern Lights (aurora  borealis)  or  the  Southern  Lights
   (aurora australis) - hereafter referred to as _a_u_r_o_r_a_e - are beautiful,
   shimmering displays of lights in the skies.  These lights have been  a
   source of wonderment and awe for centuries.  They are without a doubt,
   one of the most awesome displays of natural beauty known to man.

        Aurorae are caused by high-speed, high-energy protons  and  elec-
   trons  which  collide  with  atmospheric atoms of oxygen and nitrogen.
   These bombardments cause the gas in the ionosphere to  become  ionized
   and  give  off  photons of light.  The "fluorescing" gas is not unlike
   the gases in a fluorescent light bulb, which also become  ionized  and
   give off light when excited.  Aurorae generally form at an altitude of
   about 100 km, within the E-region  of  the  ionosphere.   Occasionally
   during  intense  auroral  storms,  the  lower  boundary of the visible
   auroral forms dips down into the D-region heights slightly  below  the
   90  km  level.   The  height at which aurorae occur enables them to be
   seen for hundreds of kilometers before the  curvature  of  the  earth,
   light  pollution,  geographical  obstructions or atmospheric anomalies
   blocks their view.

        The complete morphology of aurorae  is  complex  and  beyond  the
   scope  of  this  document.  Suffice it to say that the particles which
   penetrate into the atmosphere are  directed  by  the  Earths  magnetic
   field  and  that  the main penetration belt coincides with the auroral
   zone.  For more information, the interested reader is directed to  the
   many available books on aurorae and magnetic storms.

   _5._1.  _A_u_r_o_r_a_l _R_e_l_a_t_i_o_n_s_h_i_p _w_i_t_h _G_e_o_m_a_g_n_e_t_i_c _A_c_t_i_v_i_t_y

        Auroral activity is invariably linked with geomagnetic  activity.
   Magnetic  storms  are  always associated with auroral activity.  More-
   over, auroral activity is proportional to the  intensity  of  magnetic
   storms.   Increasingly  intense  magnetic  storms  yield  increasingly
   intense auroral activity.

        The intensity of an aurora depends on several  factors.   Auroral
   brightness,  aerial  extent, latitudinal extent, frequency of changing
   forms, pulsations and color changes are  all  used  to  determine  the
   relative  intensity  of auroral activity.  We say "relative intensity"
   because the intensity of an aurora is relative to the observer  making
   the observation, and his or her experience in doing so.

        Aurorae are most frequently seen at areas that reside in or  near



                               July 14, 1991





                                   - 32 -


   the auroral zone, a boundary where aurorae form most frequently.  Glo-
   bal geomagnetic activity is also highest in this zone.  The  locus  of
   auroral  activity  has  been  determined  to reside near a geomagnetic
   latitude of about 67 degrees.  Areas between approximately 65  and  70
   degrees geomagnetic latitude are generally considered to be within the
   auroral zone (with some diurnal exceptions  which  will  not  be  con-
   sidered here).

        The auroral zone contains the  _e_l_e_c_t_r_o_j_e_t,  an  area  within  the
   auroral  zone  where high electrical currents surge through the ionos-
   pheric and magnetospheric regions.  This electrojet is responsible for
   the  majority  of  magnetic  perturbations which occur in that region.
   The particularly strong anomalous behavior of the electrojet (as  well
   as  other  current  systems) during magnetic storms is what causes the
   intense magnetic fluctuations which  are  observed  in  and  near  the
   auroral  zone.   Even during periods of quiet magnetic activity, fluc-
   tuations in the auroral zone can be many times greater  than  fluctua-
   tions outside of the zone.

        It is now clear that the auroral zone carries more  meaning  than
   simply the definition of the zone where aurorae occur most frequently.
   It is also the zone where magnetic activity is highest, where particle
   penetration  into  the  atmosphere peaks, where anomalies of the iono-
   sphere are most severe, and  where  atmospheric  electrical  induction
   becomes most pronounced.

        Auroral activity in the auroral zone does not usually become dis-
   tinctly  visible  until  the  geomagnetic field becomes unsettled. The
   threshold for observing auroral  activity  increases  with  increasing
   distance  equatorward  of the auroral zone.  For example, middle lati-
   tudes generally require at least active geomagnetic conditions  before
   any  auroral activity can be discerned over the horizon.  Minor storm-
   ing usually provides good opportunities for  auroral  observations  at
   middle  and  high latitudes.  Low latitudes are generally incapable of
   viewing the auroral activity until major to severe geomagnetic  storms
   occur.  During periods of major geomagnetic storming, the auroral zone
   migrates equatorward and often resides over the Canada/U.S. border and
   into  the  northern  U.S..   These periods are usually associated with
   sustained K-indices of six or more over the  middle  latitudes.   With
   increasing activity, the visibility of auroral activity becomes possi-
   ble at progressively lower latitudes.

        It should be noted that the behavior of the southern auroral zone
   is  no  different than the northern auroral zone.  Therefore, areas of
   Australia,  New  Zealand,  etc.,  can  apply   these   characteristics
   equivalently.

   _5._2.  _S_i_g_n_i_f_i_c_a_n_c_e _o_f _A_u_r_o_r_a_e _t_o _A_s_t_r_o_n_o_m_e_r_s

        Considering the intrinsic brightness of aurorae, their occurrence
   can  be  an  annoyance to astronomers.  Bright aurorae associated with
   strong magnetic activity can obscure most of the sky.  Moreover, their
   brightnesses  can easily exceed the brightness of most stars.  Aurorae
   therefore, pose as a threat to the observing astronomer.



                               July 14, 1991





                                   - 33 -


        Astronomers usually attempt to get as high above  the  atmosphere
   as  possible  to observe stars.  However, even above all of the clouds
   and major atmospheric constituents, auroral  activity  can  remain  an
   annoying  interference  since their occurrence in the atmosphere is at
   an altitude of between 90 km and several hundred km's.  Luckily,  how-
   ever,  most  of  the  high-altitude  observing  sites  are in the low-
   latitude regions, where aurorae occur relatively infrequently.

        Aurorae can, on the other hand, be a real treat for the  astrono-
   mer who searches for them and enjoys observing them.  Aurorae can pro-
   vide a significant amount of excitement.  The activity in  aurorae  is
   often  remarkable.   Huge  and  rapid changes in color, brightness and
   form can all  contribute  to  the  spectacular  events  which  can  be
   observed  in  aurorae.   Activity peaks when aurorae are seen directly
   overhead.  Large, wavelike pulsations of light become  easily  visible
   when  seen  overhead.  These _f_l_a_m_i_n_g _a_u_r_o_r_a are often intensely bright
   and are constantly in motion.  Bursts of auroral activity  (associated
   with  magnetospheric  substorms) can dramatically increase the bright-
   ness and intensity of auroral activity within minutes.   The  combined
   brightness  of  auroral  activity  during intense auroral storms often
   surpasses the light given off by the full-moon.  It is no wonder  many
   astronomers often greet auroral activity with smiles and cheers.

   _5._3.  _A_u_r_o_r_a_l _C_l_a_s_s_i_f_i_c_a_t_i_o_n_s

        There are several ways of classifying aurorae.  They can be clas-
   sified  according  to shape, brightness, activity and even color.  For
   most  purposes,  however,  classifications  according  to  shape   and
   activity are enough.

        Aurorae can occur in a near-infinite number of shapes and  sizes.
   There  are,  however, forms which are more commonly seen.  These forms
   have been given names to help identify them.

        The _z_e_n_i_t_h _a_u_r_o_r_a_e is best known near and in  the  auroral  zones
   where  aurora  are seen throughout the sky, and directly overhead.  As
   it implies, zenith aurorae are aurorae which occur directly  overhead.
   They  appear  as  a  closely packed cluster of "beams" or "rays" which
   often change rapidly in shape, brightness and orientation.  They often
   appear  almost  three-dimensional and are one of the more active forms
   of aurorae.  The color of zenith aurorae vary considerably with  time.
   Rapid  and  intense color fluctuations are often associated with these
   type of aurorae.

        A well known auroral form is the _c_u_r_t_a_i_n _a_u_r_o_r_a.   These  aurorae
   are  observed  away from the zenith (either to the north or the south)
   and resemble curtains or drapes hung from the sky.  They often  change
   in shape moderately quickly.  Particularly intense segments of curtain
   aurorae often drift eastward or westwards.  The direction of drift  is
   closely  related  to  the time that the observations are made.  Unlike
   the zenith aurorae, curtain aurorae are a relatively stable form  that
   may  persist  for  hours (although their shapes may change continually
   throughout their existence).  The color of curtain aurorae  vary,  but
   are most often seen as greenish-white with occasional tinges of red or



                               July 14, 1991





                                   - 34 -


   pink.

        Closely related to the curtain  aurora  is  the  _f_l_a_m_i_n_g  _a_u_r_o_r_a.
   Flaming  aurora  are basically curtain aurora which pulsate rapidly in
   brightness.  The pulsations take on  wave-like  characteristics  which
   resemble  flames  of fire.  The wavelike pulsations propagate from the
   curtain aurora upward toward the zenith from all  directions.   Often,
   these  pulsations  converge at the zenith where diffuse aurora of pul-
   sating shapes become visible.  The flaming aurora have  been  mistaken
   for  huge  fires  occurring in distant lands by people in the times of
   the Roman Empire.  There was one instance where a Roman  Emperor  sent
   out  men  and equipment to find and extinguish a fire they thought had
   engulfed a distant castle.  Little did they know that the fire  was  a
   flaming aurora associated with a strong magnetic storm.

        The _p_u_l_s_a_t_i_n_g _a_u_r_o_r_a is a general term applied to auroral  shapes
   which  exhibit  pulsations.   Pulsating  aurora do not generally occur
   until geomagnetic activity reaches minor to major storm levels.   They
   are  characteristics  of  intense  ionospheric  ionization and tend to
   coincide closely with magnetospheric substorms (ie. periods of intense
   magnetic fluctuations and enhanced auroral activity).

        _D_i_f_f_u_s_e _a_u_r_o_r_a_e are most  prominent  during  periods  of  low  to
   moderate  geomagnetic activity.  They are usually the first to be seen
   prior to auroral and magnetospheric storms.  During  periods  of  per-
   sistent magnetic activity, diffuse aurorae may remain visible for days
   over the horizon.  High latitudes are usually able to discern  shapes,
   patterns  and  or  slight  pulsations  in  diffuse  aurorae,  but such
   activity is usually of low intensity.  These types of aurora are  gen-
   erally inactive and dull forms of auroral activity.

        Auroral _a_r_c_s are moderately bright ropes of light that arc across
   the  sky.  They can form near the boundary of the auroral zone and the
   subauroral zone (the region just outside of the auroral  zone).   Arcs
   are generally relatively inactive and don't usually exhibit pulsations
   or rapid color fluctuations.  They do, however,  undergo  occasionally
   large  changes in brightness.  The brightness intensifications usually
   precede periods of enhanced auroral and magnetic activity.   The  arcs
   are  therefore,  often  good  for  indicating  when  enhanced  auroral
   activity might be expected.  The time between an arc  brightening  and
   enhanced auroral activity may range from under less than one minute to
   more than five minutes.  Their brightenings are, however, well  corre-
   lated  with increased auroral and geomagnetic activity coinciding with
   magnetic substorms.

        These are the major forms of auroral activity which are observed.
   Although these definitions do not nearly encompass all of the possible
   forms of auroral activity (each auroral event can differ from others),
   they  encompass  most of the major types of common auroral structures.
   For a definition of the classification of  auroral  activity,  consult
   the  document  "Glossary  of  Solar  Terrestrial Terms" available upon
   request from: [e-mail].





                               July 14, 1991





                                   - 35 -


   _6.  _T_h_e _I_m_p_a_c_t_s _o_f _G_e_o_m_a_g_n_e_t_i_c _S_t_o_r_m_s _a_n_d _S_o_l_a_r _A_c_t_i_v_i_t_y

        Severe geomagnetic storms are  relatively  rare,  occurring  most
   frequently  during the maximum of the solar cycle and least frequently
   during the minimum of solar activity.  They  are  strongly  correlated
   with major solar flares, which explains their solar cycle dependence.

        Magnetic fluctuations during severe geomagnetic storms often sur-
   passes 2,000 nanoteslas (gammas), which is the smallest, most commonly
   used unit of measuring magnetic field  strengths.   Fluctuations  this
   large  over a period of minutes is enough to cause significant effects
   to terrestrial ground-based systems.  Industries which can be hit par-
   ticularly hard are the electrical generation utilities, communications
   networks, and companies managing large pipelines or other long conduc-
   tive objects.  Recent research is also revealing a causitive relation-
   ship between large geomagnetic storms and changes in atmospheric  cir-
   culation.

        In the following sections, we will attempt to cover some  of  the
   relationships between strong geomagnetic storms and impacts with these
   terrestrial systems.  We will also point out some of the  more  impor-
   tant  research which has been done with regards to solar and geophysi-
   cal activity on atmospheric circulation.  It should be noted that some
   of  the  following  material  may be considered inconclusive and still
   under research.

        The reader is warned that the material  which  follows  is  of  a
   technical  nature  and  therefore  may  not be clearly understood.  An
   attempt will be made to pad the discussion with sufficient  references
   to provide a respectable background of information with regards to the
   following discussions.  Please note that the following material is not
   essential  to  the understanding of the solar terrestrial reports.  It
   may, therefore, be skipped by those who  are  not  interested  in  the
   potential  impacts  of  solar  and geophysical activity on terrestrial
   systems and the environment.

        The discussion below has been separated into two  main  sections.
   The first section discusses the impact of magnetic storms on very long
   ground-based conductive objects such as electrical  powerlines,  pipe-
   lines,  railway networks and telecommunications networks.  The princi-
   ples discussed apply to most of these fields.  Emphasis is  placed  on
   the  electrical  power  generation industry, which can strongly affect
   the terrestrial community as a whole.  The  second  section  discusses
   the impact of severe magnetic storms and strong solar flares on atmos-
   pheric circulation, which is still in a "gray" area  with  regards  to
   conclusiveness.

   _6._1.  _M_a_g_n_e_t_i_c _S_t_o_r_m _I_n_d_u_c_t_i_o_n

        The principle  by  which  intense  magnetic  fluctuations  induce
   currents  into  long  conductive  objects has been extensively studied
   over the last several decades.  The principles are well understood and
   have been extensively verified by numerous researchers.




                               July 14, 1991





                                   - 36 -


        During major to severe geomagnetic storms, the geomagnetic  field
   exhibits  very  strong  fluctuations in intensity.  These fluctuations
   are caused by strong electrical currents residing  in  the  ionosphere
   and  deep  inside the Earth.  During these storms, electrical currents
   are able to flow through the grounded  neutral  lead  of  large  power
   transformers and into the power system.  These induced currents in the
   neutral lead causes additional magnetic fields to develope and  reside
   in  the  core of these large transformers.  The presence of these mag-
   netic fields in the core of the transformer produce spikes in  the  AC
   waveform in the transformer (caused by the addition of the normal mag-
   netic fields with the induced magnetic fields).  These spikes  produce
   harmonics  which  can  trip  protective  relays.   They also cause the
   transformer to operate less efficiently.  This lack of efficiency  can
   significantly increase the amount of current drawn by the transformer,
   effectively placing an additional load on the power  system.   If  the
   harmonics  occur for a sufficiently long period of time, physical dam-
   age to the transformer can occur.

        For example, the major magnetic storm of March 13  and  14,  1989
   induced  electrical currents on many of the electrical power distribu-
   tion networks in Canada.  Induced currents measured by  Ontario  Hydro
   during  this  storm  were  about  80  amperes/phase.  Newfoundland and
   Labrador Hydro Electric Utilities  witnessed  geomagnetically  induced
   currents as high as 150 amps/phase.  Hydro Quebec experienced magneti-
   cally induced currents powerful enough to saturate transformers.   The
   transformer  saturations  generated harmonics which tripped protective
   relays on static compensators.  The loss  of  power  caused  by  these
   events  (of  near  9,450  Megawatts) overloaded the rest of the system
   within seconds and resulted in a total collapse.   The  ensuing  power
   blackout  lasted about nine hours and affected over six million people
   in Quebec.  This storm had many effects on the electrical power indus-
   try.   Many  stations experienced numerous power fluctuations, voltage
   depressions and surges.

        The effects of geomagnetic storms on long conductive objects have
   been  studied  since  the beginning of this century.  Since then, many
   authors have elaborated on the characteristics and principles  whereby
   such  phenomena  occur.  For a good (although technical) discussion of
   these principles and characteristics,  consult  the  papers  by  Camp-
   bell[5] , Watanabe and Shier[6] , Anderson et al.[7] , Lanzerotti  and
   Gregori[8] , P.R. Barnes and J.W. Van Dyke[9] , D.H. Boteler[10] , and
   _________________________
     [5] (1986) An interpretation of induced electric  currents  in  long
   pipelines  caused  by  natural  geomagnetic sources of the upper atmo-
   sphere; Surveys in Geophysics, vol. 8, pages 239-259.
     [6] (1982) Magnetic storm effects  on  power  transmission  systems;
   Geomagnetic Bulletin, no. 2-82, Earth Physics Branch, Ottawa.
     [7] (1974) The effects of geomagnetic  storms  on  electrical  power
   systems;  IEEE  Transactions on Power Apparatus and Systems, vol. PAS-
   93, no. 4, pages 1030-1044.
     [8] (1986)  Telluric  currents: the natural environment and interac-
   tions with man-made systems; in  The  Earths  Electrical  Environment,



                               July 14, 1991





                                   - 37 -


   again by D.H. Boteler.[11]

        In previous years, telecommunications cables have been damaged by
   magnetic storms.  Damage was reported in 1958 and again in 1972 during
   severe geomagnetic storms.  These lines were made of conductive  metal
   and  carried magnetically-induced currents through the lines to equip-
   ment connected to them.  The damage sustained in  previous  years  has
   been  large, despite methods to protect them against induced currents.
   Recently however,  transatlantic  telecommunications  cable  has  been
   replaced with fibre-optic lines, which are not conductive.  During the
   major magnetic storm of March 1989, the fibre-optic cable itself  sus-
   tained  no  damage  and  experienced no problems.  However, the power-
   supply lines which accompany the fibre-optic cables  and  are  conduc-
   tive,  sustained  damaging  voltage surges as high as 700 volts during
   the March 1989 magnetic storm.

        Pipelines experience the same kinds of damaging effects as  occur
   on  powerlines and telecommunications cables.  Protective equipment on
   pipelines are used to prevent rogue surges from damaging the pipelines
   through  excessive  electrolytic corrosion at weak points in the pipe-
   line coating.  During the March 1989 storm, these  protective  systems
   were  rendered useless on many pipelines due to the excessive currents
   which were produced during the  storm.   Some  electrolytic  corrosion
   undoubtably occurred on many pipelines as a result.

        The effects of strong geomagnetic storms on  terrestrial  systems
   is  well  known.   The  power and magnitude of their influence can, at
   times, be remarkable (as was manifest by the large power  blackout  in
   Quebec  during the last severe global geomagnetic storm).  Industry is
   slowly devising ways and equipment to cope with strong  magnetic  per-
   turbations,  but  is  still  a  long  ways  away from immunity to such
   natural events.

   _6._2.  _A_t_m_o_s_p_h_e_r_i_c _C_i_r_c_u_l_a_t_i_o_n _M_o_d_i_f_i_c_a_t_i_o_n_s

        For  decades,  researchers  have  been  attempting  to  determine
   whether  large  solar  events  and  correspondingly  large geophysical
   activity affect the global atmospheric circulation of  the  earth.   A
   great  deal  of  research  has  been done in this respect, and further
   research is still needed in order to qualitatively  confirm  anomalies
   produced  by  any  geophysical or solar activity.  In this section, we
   will touch on some of the aspects of geophysical  and  solar  activity
   which apparently have been well-correlated with changes in atmospheric
   circulation.  The physical mechanisms for such changes  are  not  well
   _________________________
   U.S. NRC Report.
     [9] (November 1990) Economic Consequences of Geomagnetic  Storms  (a
   summary); IEEE Power Engineering Review, November 1990.
     [10] (1979) The Problem of Solar Induced Currents; presented at  the
   I.S.T.P.  Workshop in Boulder, Colorado in April, 1979.
     [11] (1991) Predicting Geomagnetic Disturbances  on  Power  Systems;
   EOS, April 2, 1991.



                               July 14, 1991





                                   - 38 -


   known, and certainly in many cases are still heavily  disputed.   How-
   ever,  the correlations achieved in previous research cannot be easily
   dismissed.  We therefore, expect the reader to understand  the  nature
   of this section and treat it as inconclusive, yet correlated evidence.
   For more information, we trust the interested reader will consult  the
   papers and publications cited herein.

   _6._2._1.  _A_t_m_o_s_p_h_e_r_i_c _P_r_e_s_s_u_r_e _R_e_s_p_o_n_s_e_s _t_o _S_o_l_a_r _F_l_a_r_e_s

        A pronounced cellular structure of pressure change was discovered
   by Schuurmans[12] , who calculated the difference in the 500 mb height
   before  and  after a major flare.  A total of 53 cases were originally
   studied, which was later  expanded  to  81  cases  by  Schuurmans  and
   Oort.[13] The flare threshold level was chosen to be of optical  class
   2B  or greater.  Flares of class 2B or greater were therefore included
   in this study.  Data from 1020 observation locations were used to pro-
   vide  coverage  of  most  of  the  northern  hemisphere.   Regions  of
   increased 500 mb height rise were observed near  the  longitudes  50W,
   115W,  150W,  165E, 135E, and 5E.  Height decreases were observed near
   35W, 175W, 145E, and 85E.  The most pronounced changes were  areas  in
   the  middle  latitude zones (40 to 60 degrees) with cellular groupings
   most apparent near the coastal regions.  The height  differences  were
   observed to be mostly negative poleward of about 70 degrees latitude.

        The apparent cellular  structure  of  pressure  change  following
   major solar flares was also detected in studies performed by Duell and
   Duell.[14] Using data collected by Duell  and  Duell,  Schuurmans  and
   Oort performed a critical statistical analysis on the accumulated data
   and concluded that "the central values in the  main  areas  of  height
   fall  and height rise are probably meaningful and thus not due to pure
   chance."

        Schuurmans and Oort continued with an analysis  of  the  pressure
   changes  which  occurred  in the vertical plane before and after major
   flares of class 2B or greater.  They found that maximum flare response
   was  found  to occur at the 300 mb level, at least along the 60 degree
   north latitude parallel between longitudes of approximately  0  to  70
   degrees  west.   The greatest average change of +4.7 gpdm was found at
   the 300 mb level over the North Atlantic by a ship positioned at  56.6
   N,  51.0 W.  At higher elevations, maximum response was noted to occur
   approximately six hours after  flare  time.   At  the  Earths  surface
   (approx.  1000  mb),  the atmospheric changes lagged the flare time by
   about two days.
   _________________________
     [12] (1965) Influence of solar flare particles on the general circu-
   lations of the atmosphere.  Nature, no. 205, beginning on page 167.
     [13] (1969)  A  statistical  study of pressure changes in the tropo-
   sphere and lower stratosphere after strong solar flares. Pure  Applied
   Geophysics, no. 75, pages 233-246.
     [14] (1948) The behavior of atmospheric pressure  during  and  after
   solar particle invasions and solar ultraviolet invasions.  Smithsonian
   Miscellaneous Collection 110, no. 8.



                               July 14, 1991





                                   - 39 -


        Along with the pressure-height changes which were  observed  over
   the  North Atlantic regions, a fairly significant change in the verti-
   cal temperature distribution was also observed over these regions.   A
   maximum change of near +1.1 degrees Celcius was observed at the 500 mb
   level, and a maximum  decrease  of  about  -1.8  degrees  Celcius  was
   observed  at  the  200  mb level.  The strongest temperature gradients
   were observed near the 300 mb level where the change in  pressure  was
   greatest.

        The speed of the geostrophic wind flow increased notably  at  the
   500  mb  level  in  latitudes from 55 to 75 degrees north by about 0.5
   m/s.  Near the 50 degree north latitude  zone,  a  decrease  in  geos-
   trophic wind flow by about 0.4 m/s was observed.

        Seasonally, the cellular structure which was found by  Schuurmans
   and  Oort changes very little.  However, the largest changes in height
   were found in the winter and the smallest changes were observed during
   the summer.

        Considering the large changes in pressure  at  the  8  km  height
   level  down to the surface over the North Atlantic, formed after major
   flares, one would expect a mass transport of air downward.

        In an attempt to  determine  the  validity  of  this  hypothesis,
   Reiter[15] measured the concentrations of  tracer  elements  Be^7  and
   P^32  at  Zugspitze,  which is located at an elevation of 2.96 km.  He
   found significantly increased concentrations of these elements on  the
   second  day  following  major  flares  of  importance  2B  or greater.
   According to Reiter, these two radioactive nuclides are formed in  the
   stratosphere  by  cosmic ray spallation and their increased concentra-
   tions at Zugspitze is an indication of a mass  transport  of  stratos-
   pheric air.  Reiter noted that the possibility of increased concentra-
   tions of the tracer elements at Zugspitze was not likely to have  been
   generated  by  in  situ production by enhanced solar cosmic ray fluxes
   associated with the flares, because the production rate would be  ord-
   ers  of magnitude too small to explain the observed nuclide concentra-
   tions.  Furthermore, he noted that the  maximum  concentrations  coin-
   cided  with  maximums  in solar wind velocity and geomagnetic activity
   following the larger flares.  This coincides nicely with  the  average
   arrival  time  of  large interplanetary shockwaves for major flares of
   class 2B or greater.

   _6._2._2.  _A_t_m_o_s_p_h_e_r_i_c _E_l_e_c_t_r_i_c_a_l _E_n_h_a_n_c_e_m_e_n_t_s _f_o_l_l_o_w_i_n_g _M_a_j_o_r _F_l_a_r_e_s

        Observations and measurements of atmospheric  electrical  proper-
   ties  were  made  during  70  major  flares  between  1956 and 1959 by
   Reiter.[16] Other investigations have been performed by Holzworth  and
   _________________________
     [15] (1973) Increased influx of stratospheric  air  into  the  lower
   troposphere after solar H-alpha and X-ray flares. Journal of Geophysi-
   cal Research, #78, page 6167.
     [16] (1969) Solar flares and their impact on potential gradient  and
   air-earth current characteristics at high mountain stations.  Pure Ap-



                               July 14, 1991





                                   - 40 -


   Mozer[17], Bossolasco et al.[18] [19], Markson[20], Herman  and  Gold-
   berg[21] [22], Cobb[23], and Reiter.[24] [25]

        Reiter, at the Zugspitze observatory, found that both the  poten-
   tial  gradient  and  the air-earth current density increased beginning
   shortly after a major flare.  The values peaked between 3 and  4  days
   after the flare.

        Measurements conducted by Cobb on Mauna Loa mountain in Hawaii  a
   few  years  earlier  indicated  a sharp increase in both the potential
   gradient and the air-earth current density following solar flares  and
   remained  above  normal  for  several days thereafter.  Cobb's peak in
   potential gradient occurred at about the same time as Reiter's, 3 to 4
   days  after the major flares, but his air-earth current density peaked
   only one day after the flare.

        It should be noted that these observations, by Reiter  and  Cobb,
   were performed at altitudes above the mixing layer where the potential
   gradient and air-earth current densities do  not  undergo  any  large,
   localized fluctuations.  Therefore, variations in these two parameters
   should reflect changes on a global scale.

        The atmospheric electrical changes which appear  to  occur  after
   solar  flares  leads  to  the  question  of  whether the occurrence of
   _________________________
   plied Geophysics, #72, pages 259-267.
     [17] (1977) Direct evidence of solar flare effects on weather relat-
   ed electric fields at balloon altitudes. Eos #58, page 402.
     [18] (1972) Solar flare control of thunderstorm activity,  in  Studi
   in  onore  di  G.  Aliverti, Instituto Universitario Navale Di Napoli,
   page 213.
     [19] (1973) Thunderstorm activity and interplanetary magnetic field.
   Revista Italiana di Geofisica #12, page 293.
     [20] (1971) Considerations regarding solar and lunar  modulation  of
   geophysical  parameters,  atmospheric  electricity, and thunderstorms.
   Pure Applied Geophysics, #84, page 161.
     [21] (1976)  Solar  activity  and thunderstorm occurrence.  Eos #57,
   page 971.
     [22] (1978) Initiation of non-tropical thunderstorms  by  solar  ac-
   tivity.  Journal of Atmospheric Terrestrial Physics, #40, page 121.
     [23] (1967) Evidence of a solar influence on the  atmospheric  elec-
   tric  elements at Mauna Loa Observatory.  Monthly Weather Review, #95,
   page 12.
     [24] (1971) Further evidence for impact of solar flares on potential
   gradient  and  air-earth current characteristics at high mountain sta-
   tions.  Pure Applied Geophysics, #86, pages 142-158.
     [25] (1972) Case study concerning the impact of solar activity  upon
   potential  gradient  and  air-earth  current in the lower troposphere.
   Pure Applied Geophysics, #94, pages 218-225



                               July 14, 1991





                                   - 41 -


   lightning frequency increases after a solar flare.   With  respect  to
   this, Reiter noted a 57% increase in sferics counts maximizing about 4
   days after flare-day during the years 1964 to 1967.  When compared  to
   Reiters  results  regarding  the  potential  gradient  over these same
   years, it is found that the magnitude of increases in  sferics  counts
   and in the potential gradient are comparable.

        Markson (1971) analyzed the occurrence frequency of thunderstorms
   with  solar  flares in the United States for the sunspot minimum years
   1964 to 1965.  He found a 63% increase in occurrence frequency  maxim-
   izing  about  7  days  after flare eruptions.  He pointed out that his
   maximum in the U.S. occurred about 3 days after the maximum in  poten-
   tial gradient found by Reiter at Zugspitze.  This long lag time there-
   fore makes it uncertain (at least, based on  these  results),  whether
   United  States thunderstorm activity is affected by solar activity the
   same as in the regions observed by Reiter.

        On a globabl basis, Bossolasco et  al.  found  that  thunderstorm
   activity  increased  by 50% in solar minimum years and by 70% in solar
   maximum years about 4 days after flare eruptions.   The  frequency  of
   lightning  strikes  in the Mediterranean area was observed to increase
   markedly about 4 days  after  the  eruption  of  large  solar  flares.
   Through superposed epoch analysis of the data in the foregoing, it has
   been established  that  the  occurrence  frequency  begins  a  notable
   increase  one day after the flare event and achieves a 50% increase on
   the 4th day.  These results are in good agreement with those  obtained
   by Reiter at the Zugspitze observatory.

        Data analyzed over a full solar cycle (between the years 1961 and
   1971)  exhibited  the same results, as determined by Bossolasco et al.
   (1973).

        From these results, it appears that the  air-earth  current  den-
   sity,  ionospheric  potential, potential gradient and the frequency of
   lightning strikes responds to solar  flares.   Enhancements  in  these
   quantities  occur  between  1 and 4 days after the flare eruption with
   the increase in lightning frequency responding the slowest.

        A suggested possible physical mechanism  lies  in  the  increased
   potential gradient around the 20 km altitude level.  High energy solar
   protons ejected from major lares penetrate the atmosphere down to lev-
   els as low as 20 km.  The increased ionization at these levels (during
   intense events) enhance the conductivity above about 20 km.  Below  20
   km,  Forbush  decreases  in  cosmic ray intensity results in decreased
   conductivity.  The potential gradient and  ionospheric  potential  are
   also  alertered  and the net result is a possible increase in thunder-
   storm activity.

   _6._2._3.  _G_e_o_m_a_g_n_e_t_i_c _E_f_f_e_c_t_s _o_n _A_t_m_o_s_p_h_e_r_i_c _P_r_e_s_s_u_r_e

        Based on an analysis of low-pressure trough  development  at  the
   300  mb  level  in  the  North Pacific and North America areas for the
   years 1956-1959, Macdonald and Roberts[26] found that, in  the  winter
   _________________________



                               July 14, 1991





                                   - 42 -


   seasons, 300 mb troughs entering or forming in the Gulf of Alaska area
   2 to 4 days after a major geomagnetic storm are likely to undergo much
   greater deepening than those entering at other times.   Macdonald  and
   Roberts[27] as well as Twitchell[28] verified  that  these  conditions
   are also manifest at the 500 mb level.

        Roberts and  Olson[29],  using  a  vorticity  area  index  (VAI),
   extended these earlier results.  They defined the VAI as the area of a
   trough wherein the absolute vorticity was greater  than  or  equal  to
   20(10^-5)/second  summed  with  the area where it is 24(10^-5)/second.
   This index removes the  subjectiveness  from  the  assessment  of  the
   intensity and importance of troughs and the minimum threshold vortici-
   ties for the definition were selected as such because most  wintertime
   300  mb troughs exceed a vorticity of 20(10^-5)/second, and large ones
   have a substantial region exceeding the largest vorticity value.

        The results obtained by Roberts and Olson confirmed  the  earlier
   findings of Macdonald and Roberts.  Using data spanning the years 1964
   to 1971, Roberts and Olson found that there are two statistically sig-
   nificant periods of time when key troughs undergo a sharp rise in vor-
   ticity area index.  The first occurs during the first  three  days  of
   trough lifetime.  On the average, this occurs three to five days after
   the start of a geomagnetic storm.  It is important to note that  their
   findings  showed that 2 to 4 days must elapse between the beginning of
   a geomagnetic storm and the appearance of the trough in order for  the
   effect to be observed.  On occasions when less than 2 days elapsed, no
   VAI intensification occurred (as was  later  discovered  by  Olson  et
   al.[30]). The second statistically significant period  of  time  where
   troughs  undergo  significant  increases  in  VAI occurs about 10 days
   after geomagnetic storms.  Asakura and  Katayama[31]  also  discovered
   significant  decreases  in  pressure  and  increased cyclogenesis over
   north-eastern coastal regions of North America.
   _________________________
     [26] (1960) Further evidence of a  solar  corpuscular  influence  on
   large-scale  circulation  at 300 mb.  Journal of Geophysical Research,
   #65, pages 529-534.
     [27] (1961) The effect of solar corpuscular emission on the develop-
   ment of large troughs in the atmosphere.  Journal of Meteorology, #18,
   pages 116-118.
     [28] (1963) Geomagnetic storms and 500 mb trough behavior.  Bulletin
   of Geophysics, #13, pages 69-84.
     [29] (1973) Geomagnetic storms and wintertime 300 mb trough develop-
   ment  in the North Pacific-North America area.  Journal of Atmospheric
   Science, #30, page 135.
     [30] (1975)  Short  term relationships between solar flares, geomag-
   netic storms, and tropospheric vorticity patterns.  Nature, #257, page
   113.
     [31] (1958) On the relationship between solar activity  and  general
   circulation  of  the  atmosphere.  Meteorological Geophysics, #9, page
   15.




                               July 14, 1991





                                   - 43 -


        Reitan[32] noted, after analyzing data over  the  20-year  period
   1951-1970, that the distribution of cyclonic event occurrence in Janu-
   ary over the northern hemisphere exhibited a maximum in the  areas  of
   the  Gulf  of Alaska and the northeastern coastal region of the United
   States.  These are also  the  areas  where  Roberts  and  Olson  found
   increases in VAI following geomagnetic storms.  A correlation analysis
   was performed to analyze the association  of  SSC-related  geomagnetic
   storm  occurrences  and  the number of cyclonic events observed in the
   United States over the  period  1951-1967,  by  Mayaud[33].  What  was
   discovered  was  a  statistically  significant  (94% confidence level)
   correlation coefficient of -.46 between SSC-related geomagnetic storms
   and  the  number  of  cyclonic  events observed in the U.S. during the
   period.  These results, combined with those of  Roberts  and  Olson  ,
   suggest that, although fewer cyclonic events may occur during the sun-
   spot maximum years, they are larger and more  intense  than  the  more
   numerous ones that form in the solar minimum years.

        From the data which has thus far been accumulated, it appears  as
   though  the  strongest  meteorological  effects  of  solar  flares and
   geomagnetic storms occurs during the winter  season  in  the  northern
   hemisphere.   Although the data contained in this document just barely
   scratches the surface of research which has been done over the  years,
   there are still doubts whether a solar or magnetic link to terrestrial
   atmospheric circulation patterns actually exists.  It is  our  impres-
   sion  that  such  a  link may indeed exist, but additional research is
   needed in order to determine the areas and physical  mechanisms  which
   link solar and/or geomagnetic activity to specific atmospheric events.
   Nevertheless, the research data which has accumulated over  the  years
   cannot  be dismissed, for there are a great many relationships between
   solar activity, geomagnetic activity and atmospheric  phenomena  which
   appear to have strong correlations.

        Those persons with sufficient background who  are  interested  in
   obtaining  more information regarding the possible influences of solar
   activity on terrestrial atmospheric processes, are directed to  obtain
   the  book "Sun, Weather, and Climate" by John R. Herman and Richard A.
   Goldberg (formerly published as NASA SP-426 but  recently  republished
   by  Dover  Publications Inc. in book form).  This document nicely sum-
   marizes most of the research which has been done in this area over the
   years and provides some convincing evidence between solar, geomagnetic
   and atmospheric  relationships.   For  more  recent  information,  the
   interested reader is encouraged to browse through the various journals
   covering this subject and the published results of numerous solar ter-
   restrial workshops and symposiums.


   _________________________
     [32] (1974) Frequencies of cyclones and cyclogenesis for North Amer-
   ica, 1951-1970.  Monthly Weather Review, #102(12), page 861.
     [33] (1973) A 100-year series of geomagnetic data: Indices aa, storm
   sudden  commencements.   IAGA Bulletin 33, International Union of Geo-
   detic Geophysics, Paris.




                               July 14, 1991





                                   - 44 -


   _7.  _C_o_n_c_l_u_d_i_n_g _R_e_m_a_r_k_s

        There are many aspects of solar physics and  geophysics  (not  to
   mention  atmospheric  physics) which must be understood before a clear
   knowledge of the interactions between solar activity  and  terrestrial
   phenomena  can be obtained.  This document was prepared to aid in pro-
   viding  the  most  basic  and  fundamental  characteristics  of  solar
   activity  and geophysical phenomena required to understand and respect
   the nature of the solar terrestrial reports which are posted over  the
   networks.

        This document was intended to be  understood  by  those  who  are
   unfamiliar  with  solar  terrestrial  physics.  The  solar terrestrial
   reports posted over the networks are in as simple a form as is practi-
   cal  without  losing  any significant resolution of information.  They
   are written in a form that should be easily understood once the  basic
   principles and language become familiar.

        The preceding presentation was required in order  to  supply  the
   interested  reader  with  the  information  and language background to
   understand the solar terrestrial reports.  Only  the  latter  sections
   were directed towards those with an interest and background in geophy-
   sics and atmospheric physics.  The rest of the  material  should  have
   been  interpretable by those whose backgrounds and/or interests lie in
   other areas.

        This document is not intended to be fully  understood  the  first
   time  through.  It should be reread and digested as necessary and used
   (if necessary) as a reference to the solar terrestrial reports.

        Now that we have the background necessary to understand the solar
   terrestrial  reports, we may begin a systematic analysis of the struc-
   ture and content of the reports themselves.  The accompanying document
   (part  II)  will describe the solar terrestrial reports in detail with
   accompanying hints and procedures that may be used to  extract  useful
   and pertinent information.





















                               July 14, 1991





                                   - 45 -





                             Table of Contents




   Introduction ....................................................    1

   Characteristics of the Sun ......................................    2

   Sunspots and the Solar Flux .....................................    3

   The Sunspot Cycle ...............................................    6

   The 22 Year Solar Cycle .........................................    6

   The Solar Atmosphere ............................................    7

   The Photosphere .................................................    7

   The Chromosphere and Spicules ...................................    8

   The Corona and Coronal Holes ....................................    8

   Forms of Solar Activity .........................................   10

   Plages and Faculae ..............................................   10

   Prominences and Filaments .......................................   11

   Solar Flares ....................................................   13

   Polar Cap Absorption Events .....................................   16

   Significance of Sweep Frequency Events ..........................   17

   The Slowly Varying Component ....................................   17

   Type I Bursts and Radio Noise Storms ............................   18

   Type III Radio Bursts ...........................................   18

   Type V Radio Burst Emissions ....................................   19

   Type II Radio Bursts ............................................   19

   Continuum Type IV Radio Emissions ...............................   20

   The Earths Magnetic Field .......................................   21

   Geomagnetic Substorms ...........................................   22




                               July 14, 1991





                                   - 46 -


   Geomagnetic Storms ..............................................   23

   Ionospheric Effects of Geomagnetic Storms .......................   24

   Radio Signal Propagation ........................................   25

   Propagation of VLF Signals ......................................   26

   HF Signal Propagation ...........................................   26

   Long-Distance VHF Signal Propagation ............................   29

   Characteristics of Auroral Activity .............................   31

   Auroral Relationship with Geomagnetic Activity ..................   31

   Significance of Aurorae to Astronomers ..........................   32

   Auroral Classifications .........................................   33

   The Impacts of Geomagnetic Storms and Solar Activity ............   35

   Magnetic Storm Induction ........................................   35

   Atmospheric Circulation Modifications ...........................   37

   Atmospheric Pressure Responses to Solar Flares ..................   38

   Atmospheric Electrical Enhancements following Major Flares ......   39

   Geomagnetic Effects on Atmospheric Pressure .....................   41

   Concluding Remarks ..............................................   44
























                               July 14, 1991