home
***
CD-ROM
|
disk
|
FTP
|
other
***
search
/
Shareware Overload
/
ShartewareOverload.cdr
/
database
/
bcast100.zip
/
USTR1.DOC
< prev
next >
Wrap
Text File
|
1991-11-18
|
158KB
|
3,084 lines
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 "oler@hg.uleth.ca".
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
July 14, 1991
- 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
July 14, 1991
- 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
July 14, 1991
- 4 -
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.
July 14, 1991
- 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
July 14, 1991
- 6 -
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
_________________________
July 14, 1991
- 7 -
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.
July 14, 1991
- 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
July 14, 1991
- 9 -
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: oler@hg.uleth.ca.
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