AOH :: SOL_GLOS.TXT|
Glossary of solar astronomical terms
Article 5607 of rec.radio.shortwave:
Comments: Gated by NETNEWS@AUVM.AMERICAN.EDU
Date: Fri, 5 Jul 91 17:19:14 MDT
Reply-To: CARY OLER <oler@HG.ULETH.CA>
Sender: Short Wave Listener's List <SWL-L@OHSTVMA.BITNET>
From: CARY OLER <oler@HG.ULETH.CA>
Subject: Glossary of Solar Terrestrial Terms
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.
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.
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
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).
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.
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
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
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.
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
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
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
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.
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
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
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.
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
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:
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.
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
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
- 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.
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.
The entire AOH site is optimized to look best in Firefox® 3 on a widescreen monitor (1440x900 or better).
Site design & layout copyright © 1986- AOH
We do not send spam. If you have received spam bearing an artofhacking.com email address, please forward it with full headers to email@example.com.