Shareware Overload/ShartewareOverload.cdr/database/bcast100.zip/GLOSSARY.DOC

Text File | 1991-11-18 | 25KB | 444 lines

                      GLOSSARY OF SOLAR TERRESTRIAL TERMS
                      -----------------------------------


     The solar terrestrial forecasts which are being distributed over the
networks contain some language that may not be very clear to many people
unfamiliar with solar terrestrial terms.  Since the reports are intended to
be intelligable by the general public, this glossary of terms has been
compiled to help provide some explanations for terms which may be used in
the reports. This glossary is not meant to be exhaustive, but is rather meant
to provide people with a well-rounded vocabulary and a basic knowledge of some
of the terms and classifications used in the reports.

Definitions are not in any particular order.


Solar Flux:

     The 10.7 cm (2800 MHz) radio flux is the amount of solar noise that is
emitted by the sun at 10.7 cm wavelengths.  The solar flux is measured and
reported at approximately 1700 UT daily by the Penticton Radio Observatory in
British Columbia, Canada.  Values are not corrected for variations resulting
from the eccentric orbit of the Earth around the Sun.  The solar flux is used
as a basic indicator of solar activity.  It can vary from values below 50 to
values in excess of 300 (representing very low solar activity and high to
very high solar activity respectively).  Values in excess of 200 occur
typical during the peak of the solar cycles.  The solar flux is closely
related to the amount of ionization taking place at F2 layer heights (heights
sensitive to long-distance radio communication).  High solar flux values
generally provide good ionization for long-distance communications at higher
than normal frequencies.  Low solar flux values can restrict the band of
frequencies which are usable for long distance communications.  The solar
flux is measured in "solar flux units" (s.f.u.).  One s.f.u. is equivalent to
10^-22 Wm^-2 Hz^-1.


Sunspot Number:

     This term is basically self-explanatory.  It represents the number of
observed sunspots and sunspot groups on the solar surface.  It is computed
according to the Wolf Sunspot Number formula: R = k (10g + s), where 'g' is the
number of sunspot groups (regions), s is the total number of individual spots
in all the groups, and k is a scaling factor that corrects for seeing
conditions at various observatories.  Sunspot number varies in phase with the
solar flux.  Sunspot numbers can vary between zero (for sunspot minimum
periods) to values in excess of 350 or 400 (in the very active "solar max"
period of the suns 11 year cycle).  Solar flux is related to the sunspot
number, since sunspots produce radio emissions at 10.7 cm wavelengths (as
well as at other wavelengths).


X-Ray Background Flux:

     This represents the average background x-ray flux as measured on the
primary GOES satellite.  This value basically represents the amount of x-ray
radiation that is being received at the Earth by the Sun.  Generally, active
regions emit more x-ray radiation than non-active solar regions.  Therefore,
this value can be of use in determining the overall state of the solar
hemisphere facing the Earth.  This value is also useful for propagation
prediction models (ie. PROPHET models), since ionospheric layer ionization
is closely correlated with the background X-ray flux.  This flux is stated
using the same classification scheme for x-ray flares (given below).


Proton Fluence:

     Although this term will seldom be referenced within the reports, it may be
of use to make a note of it.  Proton fluence is simply the total number proton
particle fluxes measured by the GOES spacecraft at geosynchronous altitudes for
protons of energies >1 Million electron Volts (MeV), >10 MeV and >100 MeV.  The
higher the proton fluence, the more intense proton bombardments are at
geosynchronous altitudes.  It can also be used implicitly to determine the
approximate amount of ionization occurring in the upper atmosphere, as well as
the proton penetration level into the atmosphere and possible satellite
anomalies caused by the solar proton bombardments.  Fluence for particles are
given in the units:  particles cm^-2 steradian^-1 day^-1.


Tenflare:
     A tenflare is associated with optical and x-ray flares.  Solar flares
emit radiation over a very wide range of frequencies.  One of the more
significant frequencies observed is the 10.7 cm wavelength band (2695 MHz).
When a solar flare erupts, "noise" from the flare is received over this very
wide range of frequencies.  When the noise received on the 10.7 cm wavelength
band surpasses 100% of the background noise level during a solar flare, a
Tenflare is said to be in progress.  The more intense solar flares are
associated with tenflares.  Almost all major flares are associated with
tenflares.  Generally, the greater the intensity of the burst of noise
observed at the 10.7 cm wavelength band, the more significant the flare is
said to be.  The duration of the tenflare can also be used to determine the
severity of the flare.  Other important flare characteristics are also
determined from the radio data observed from flares, which are closely
related to the various physical processes which occur in flares.  These
characteristics are far beyond the scope of this document.


Electron Fluence:

     Again, this term will seldom be referenced within the reports.  It is
analagous to "proton fluence" but is measured for electrons with energies
>2 MeV.  Fluence measurements are the same as those for proton fluence.


Magnetic A-Index:

     The geomagnetic A-Index represents the severity of magnetic fluctuations
occurring at local magnetic observatories.  During magnetic storms, the A-index
may reach levels as high as 100.  During severe storms, the A-index may exceed
200.  Great "rogue" storms may succeed in producing index values in excess of
300, although storms associated with indices this high are very rare indeed.
The A-index varies from observatory to observatory, since magnetic fluctuations
can be very local in nature.  The A-index for Boulder Colorado (the same value
reported on WWV and WWVH) will be the one referenced most often within the
reports.  Occassionally, the A-index for higher latitude stations may also be
referenced for purposes of comparison.  Magnetic fluctuations monitored locally
here at Solar Terrestrial Dispatch will also be referenced, particularly
during storm periods for descriptive purposes.


Magnetic K-Index:

     The geomagnetic K-Index is related to the A-index.  K-indices are scaled
by comparing the H and D magnetometer traces (representing the horizontal and
declination magnetic components) to assumed "quiet-day curves" for H and D.
Each UT day is divided into 8 three-hour intervals, starting at 0000 UT.  In
each 3-hour period, the maximum deviation from the quiet day curve is measured
for both (H and D) traces, and the largest deviation (the most disturbed trace)
is selected.  It is then input into a quasi-logarithmic transfer function to
yield the K-index for the period.  The K-index ranges from 0 to 9 and is a
dimensionless number.  It is assigned to the end of the 3 hour period.  The
K-Index is useful in determining the state of the geomagnetic field, the
quality of radio signal propagation and the condition of the ionosphere.
Generally, K index values of 0 and 1 represent Quiet magnetic conditions and
imply good radio signal propagation conditions.  Values between 2 and 4
represent Unsettled to Active magnetic conditions and generally correspond to
less-impressive radio propagation conditions.  K-index values of 5 represent
Minor Storm conditions and are usually associated with Fair to Poor propagation
on many HF paths.  K-index values of 6 generally represent Major Storm
conditions and are almost always associated with Poor radio propagation
conditions.  K-index values of 7 represent Severe Storm conditions and are
often accompanied by "radio blackout" conditions (particularly over higher
latitudes).  K-indices of 8 or 9 represent Very Severe Storm conditions and are
rarely encountered (except during exceptional periods of solar activity).
K-indices this high most often produce radio blackouts for periods lasting in
excess of 6 to 10 hours (depending upon the intensity of the event).


Sudden Storm Commencement or SSC:

     An SSC is the magnetic signature of an interplanetary shockwave most often
produced by solar flares.  It is always a precursor to increased geomagnetic
activity, most often followed within 3 to 8 hours by a Minor to Major
geomagnetic storm.  It appears on the H (horizontal) trace of magnetometers.
This phenomenon is detectable at almost all magnetic observatories world-wide
within 4 minutes of eachother.


Sudden Impulse or SI:

     A sudden magnetic impulse is similar to an SSC.  It represents a rapid
momentary fluctuation of the geomagnetic field over a period of only a few
minutes.  It is generally associated with interplanetary shockwaves produced by
energetic solar events and can (but need not always) be followed by increased
geomagnetic activity.


Satellite Proton Event:

     Proton events are almost always associated with energetic solar activity
such as major flares.  They are periods of increased proton bombardments at
satellite altitudes.  They can affect satellite transmission/reception if
intense enough and can cause other satellite anomalies.  Proton events may
affect the ability of a HAM operator to establish contact with a satellite, and
may affect the quality of television signals received by satellite (ie. cable
tv may be affected).  Satellite proton events occur within a few hours of a
major proton flare.  They are also often followed by a PCA event (see below).


Polar Cap Absorption Event or PCA:

     A PCA is almost always produced by an intense solar proton flare.  PCAs
are the result of copious quantities of high-energy solar protons penetrating
the Earths atmosphere to levels of the order of 50 km, producing intense
ionospheric ionization.  The result is a complete (or near-complete) radio
blackout over polar latitudes.  A typical PCA lasts from 1 to 5 days and can
severely effect the propagation of radio signals near or through polar regions.
In intense, long-lasting events, direct entry of the high-energy solar protons
to the upper atmosphere can extend equatorward as far as about 50 degrees
geomagnetic latitude.  They occur almost coincident with satellite-level proton
events, maximize in intensity within a few hours and then begin a slow decay
that can last up to 10 days for intense events.  A PCA is often followed within
48 hours by a SSC and a subsequent Minor to Major geomagnetic storm about 3 to
8 hours later.


Sunspot Classifications:

     Sunspots are classified according to size, shape and spot density.  They
are classified using a set of three coded letters (Zpc) as follows:

        Z - Modified Zurich class, labelled A through F plus H.
               A - Single small spot (single magnetic polarity).
               B - Very small distribution of small spots.
               C - Two or more small spots, at least one of which has a
                   detectable penumbra.
               D - Moderately sized group of spots, several of which may have
                   noticable penumbrae.  Magnetic complexity of D-type regions
                   are usually capable of producing C and low-intensity M-class
                   flares.
               E - Moderate to large area of a fairly complex system of
                   sunspots, several of which have noticable penumbrae and
                   good definition.  Often capable of producing minor C-class
                   as well as major M-class flares.
               F - Large to very large area of a complex system of sunspots.
                   These regions are often capable of producing major X-class
                   flares as well as numerous major M-class and many C-class
                   events (depending on their magnetic complexity).
               H - Single large to very large sunspot (not usually capable
                   of producing significant energetic events).  This type of
                   sunspot is usually manifest in the dying phase of a sunspot
                   group.

        p - Penumbra type of the largest spot in the group.
               x - Single spot.
               r - Rudimentary.
               s - Small symmetric.
               a - Small asymmetric.
               h - Large symmetric.
               k - Large asymmetric.

        c - Relative sunspot distribution or compactness of the group.
               x - Single spot.
               o - Open group (separated by quite a wide space).
               i - Intermediate (moderate sunspot compactness in the group).
               c - Compact (very dense and complex spots within the group).

Example:  A sunspot group classified as type DKO would be of moderate overall
size (that is, the region encompassing all of the sunspots within the group
would be of moderate size), the penumbra of the largest spot within the group
would be large and asymmetric in shape, and the group would be "open"
indicating that the sunspots within the region are not notably close together.


Magnetic Class:

     The magnetic class of sunspots is important in determining how potentially
volatile particular active regions may be.  Sunspots are regularly observed
using instruments capable of determining the magnetic polarity of sunspots and
active regions.  By also applying laws which have been formulated over the
years, visual observations can also be used to establish the magnetic polarity
and complexity of spot groups.  There are basically 7 magnetic types of
sunspots that are classified.  They are described as follows:

       Type A   - Alpha (single polarity spot).
            B   - Beta (bipolar spot configuration).
            G   - Gamma (atypical mixture of polarities).
            BG  - Beta-Gamma (mixture of polarities in a dominantly bipolar
                  configuration).
            D   - Delta (opposite polarity umbrae within single penumbra).
            BD  - Beta with a Delta configuration.
            BGD - Beta-Gamma with a Delta configuration.

Example:  A region labelled as having a magnetic classification of BG
indicates that the sunspot region contains a mixture of magnetic polarities,
but the dominant polarity of the group is bipolar.

Potentially very powerful and potent regions are those which have
classifications of BG, BD and BGD. As magnetic complexity increases, the
ability of an active region to spawn major energetic events likewise increases.


Solar Activity Description:

     Solar activity is described (also applicable on WWV and WWVH) according to
the number of flares which occur during the day.  Activity is basically
classified as follows:
        Very Low :  X-ray events less than class C.
             Low :  C-class x-ray events.
        Moderate :  Isolated (one to 4) M-class x-ray events.
            High :  Several (5 or more) M-class x-ray events or isolated
                    (1 to 4) M5 or greater x-ray events.
       Very High :  Several M5 or greater x-ray events.


Flare Classifications:

     Flares are classified using one of two different systems.  The first
classification ranks the event by measuring its peak x-ray intensity in the 1-8
angstrom band as measured by the GOES satellites.  This x-ray classification
offers at least two distinct advantages compared with the second system of
classification (optical): it gives a better measure of the geophysical
significance of the event and it provides an objective means of classifying
geophysically significant activity regardless of its location on the solar disk
or near the solar limb.  The classification scheme is as follows:

        Class           Peak Flux (1-8 Angstroms in Wm^-2)
          A                < 10^-7
          B                < 10^-6 but > class A
          C                < 10^-5 but > class B
          M                < 10^-4 but > class C
          X                            > 10^-4

The letter designates the order of magnitude of the peak value.  Following the
letter the measured peak value is given.  For descriptive purposes, a number
from 1.0 to 9.9 is appended to the letter designation.  The number acts as a
multiplier.  For example, a C3.2 event indicates an x-ray burst with a peak
flux of 3.2 x 10^-6 Wm^-2.  Since x-ray bursts are observed as a full-Sun
value, bursts below the x-ray background level are not discernable.  The
background drops to class A level during solar minimum; only bursts that exceed
B1.0 are classified as x-ray events.  During solar maximum, the background is
often at the class M level, and therefore class A, B and C x-ray bursts cannot
be seen.  Bursts greater than 1.2 x 10^-3 Wm^-2 may saturate the GOES
detectors.  If saturation occurs, the estimate peak flux values are reported.

The second system of classification involves a purely optical method of
observation.  A flare event is observed optically (in H-alpha light) and is
both measured for size and brightness.  This classification therefore includes
two items of information: a descriptor defining the size of the flare and a
descriptor defining the peak brightness of the flare.  They are listed below:

          Importance
          ----------
               S          -  Subflare area <= 2.0 square degrees.
               1          -  2.1  <= area <= 5.1  square degrees.
               2          -  5.2  <= area <= 12.4 square degrees.
               3          -  12.5 <= area <= 24.7 square degrees.
               4          -          area >= 24.8 square degrees.

          Brightness
          ----------
               F          -  Faint.
               N          -  Normal.
               B          -  Brilliant.

Example:  A major flare rated as a class M7.4/2B event indicates that the flare
attained a maximum x-ray intensity of 7.4 x 10^-5 Wm^-2. The "2B" portion of
this specification indicates that the flare was an importance 2 flare
(>= 5.2 and <= 12.4 square degrees) and was optically Brilliant.  This sample
flare is a powerful event.  Flares that reach x-ray levels in excess of class
M4 can begin to have an impact on the Earth.  Likewise, flares rated 2B or
greater are generally capable of influencing the Earth, particularly if
accompanied by Type II and IV radio sweeps (discussed below).


Sweep Frequency Events (Type II, III, IV and V events):

     Energetic solar events often produce characteristic radio "bursts".  These
bursts are generated by solar material plunging through the solar corona.  Type
III and type V events are caused by particles being ejected from the solar
environment at near relativistic speeds.  Type II and IV events are caused by
slower-moving solar material propagating outward at speeds varying between
approximately 800 and 1600 kilometers per second.  Type II and IV radio bursts
are of particular importance.  These sweep frequency radio events are
signatures of potentially dense solar material which has been ejected from the
solar surface.  If the region responsible for these events is well positioned,
the expelled solar material may succeed in impacting with the Earth.  Such an
impact often causes an SSC followed by Minor to Major geomagnetic storm
conditions and significantly degraded radio propagation conditions.  It is
therefore interesting to pay attention to events which cause Type II and/or IV
radio sweep events, since they may indicate the potential for increased
magnetic activity (and decreased propagation quality) within 48 hours.  It
should be noted, however, that predicting degraded terrestrial conditions is
significantly more complex than simply observing whether the energetic event
had an associated Type II or IV radio sweep.  Flare position, proton spectra,
flare size, event duration, event intensity and a host of other variables must
be analyzed before a qualitative judgement can be made.
     It should also be noted that sweep frequency radio events are capable of
producing Short Wave Fades (SWFs) and Sudden Ionospheric Disturbances (SIDs).
Depending on the severity of the event, the duration of SWFs and SIDs may
last in excess of several hours with typical values being approximately 30
minutes.  SWFs and SIDs cause absorption of radio signals (due to intense
ionization) at frequencies up to and well in excess of 500 MHz.  Microwave
continuum bursts can affect frequencies up to 30 GHz.  Frequencies in the HF
region can be completely blacked out for a period of time during intense
energetic events.


Classifications of Auroral Activity used in the Reports:

     Auroral activity is rated as either not visible, low, moderate, high,
very high or extremely high.  These classifications are defined according to
the brightness achieved by auroral activity, visual activity (ie. changes of
form or structure), whether the aurora is pulsating, and according to the
intensity and fluctuations of color in the aurora.  The various levels of
activity are defined below:

  - Not visible:  Self-explanatory.

  - Low:  Low intensity aurorae consisting mostly of diffuse, dim, and
lifeless activity.  Generally no rapid changes in form or structure are
discerned with auroral activity that is classified as "low."

  - Moderate:  Moderate intensity auroral activity which consists of diffuse
aurorae intermixed with curtain aurorae or other forms of relatively-low
activity aurorae.  Moderate activity may include beams or rays of aurorae
which travel either east or west with time.  No color fluctuations or
significant brightenings of aurorae are associated with moderate intensities.

  - High:  High intensity auroral activity is activity associated with very
bright, active displays that may exhibit changes of color and rapid
pulsations.  High activity is generally confined to curtain aurorae and
moderate-intensity pulsating aurorae.

  - Very High:  Very high intensity auroral activity is usually only
experienced over the high latitude regions where zenith aurorae and
significant auroral displays occur.  Activity classified as very high consists
of most auroral forms of activity, but the activity is always very bright and
extremely active.  Curtain aurorae may change form and color rapidly.  Zenith
aurorae may become exceedingly bright and colorful.

  - Extremely High:  Extremely high auroral activity is only rarely
encountered.  Activity at this level of intensity is most often experienced
within the middle and/or low latitude zones during significant periods of
geomagnetic activity.  The expansion of the auroral zone equatorward and
poleward produces the intense activity over regions equatorward of the normal
position of the auroral oval.  This activity usually consists of exceedingly
bright, rapidly fluctuating, strongly pulsating, color-varying auroral
activity.  Levels of auroral activity this high are usually only associated
with "rogue flares", which may occur only once or twice during a solar cycle.

The approximate latitudinal boundaries for observing aurorae (biased for
North America and Australia/New Zealand) follow.  The locations of these
boundaries for Europe will be higher than for North America.  The locations
for Asia will be correspondingly higher than for Europe.  The Southern
Hemisphere estimates are valid for Australia and New Zealand.  Locations of
the boundaries for southern areas of South America will be higher than for
Australia and New Zealand.

        NORTHERN HEMISPHERE                    SOUTHERN HEMISPHERE
  High latitudes >= 55      deg. N.  |   High latitudes >= 55      deg. S.
Middle latitudes >= 40 < 55 deg. N.  | Middle latitudes >= 30 < 55 deg. S.
   Low latitudes  < 40      deg. N.  |    Low latitudes  < 30      deg. S.



-----------------------

For a good discussion on the topic of solar flares and terrestrial impacts,
consult the book "Solar Flares" by H.J. Smith and E.V.P. Smith (publisher:
Macmillan, New York).  Although this book is a few years old (1963), it
provides an excellent knowledge base to build upon and a wealth of information
on flares in general.