home
***
CD-ROM
|
disk
|
FTP
|
other
***
search
/
Shareware Overload
/
ShartewareOverload.cdr
/
database
/
bcast100.zip
/
INTERP.DOC
< prev
next >
Wrap
Text File
|
1991-11-18
|
25KB
|
529 lines
INTERPRETING THE DATA CONTAINED IN THE DAILY
SOLAR GEOPHYSICAL DATA BROADCASTS
VERSION 1.0
Copyright (c) 1991 Solar Terrestrial Dispatch
ABSTRACT
A brief guide to interpreting the data contained in
the daily solar geophysical data broadcasts. This docu-
ment is intended to be most useful to those who are
interested in interpreting the data contained in the daily
reports for the purpose of determining current and future
HF radio propagation conditions as well as current and
possible future periods of auroral activity. It is
intended to be read and understood by those who do not
have a formal education in these or related fields.
November 13, 1991
INTERPRETING THE DATA CONTAINED IN THE DAILY
SOLAR GEOPHYSICAL DATA BROADCASTS
VERSION 1.0
Copyright (c) 1991 Solar Terrestrial Dispatch
1. Introduction
The Daily Solar Geophysical Data Brodcasts (SGDBs) provide a
wealth of information regarding the state of the Earth's auroral and
ionospheric environment. This document will attempt to provide the
reader with sufficient information to be able to better interpret the
data contained in the daily SGDBs for the purpose of determining
current and future conditions of auroral and/or ionospheric activity.
Although this document will focus on these areas of activity, the data
contained in these reports have far-reaching applications into many
other fields such as solar, space and atmospheric physics. However,
these professional applications and will not be covered here.
The reader is encouraged to read and digest the material con-
tained in the document, "Understanding Solar Terrestrial Reports".
Part I of that document is particularly useful for understanding the
language and physics behind some of the solar events which can affect
the Earth. That document has been written in lamans language and
should therefore be more easily understood by the public in general.
It will provide the reader with sufficient background information to
better understand and use the daily SGDB reports.
Also worth reading is the file "SGDB.DOC", which explains the
format of the SGDB reports line-by-line.
2. Determining HF Propagation Conditions
Almost all of the indices and forecasts presented in the daily
SGDBs may be applied to understanding and predicting the state of the
ionosphere. The ionosphere is very sensitive to changes in the
Earth's geomagnetic field. Fluctuations of the magnetic field and
phenomena which often accompany such fluctuations (ex. aurorae) can
often degrade the ability of the ionosphere to reflect HF radio sig-
nals. This in turn inhibits the propagation of radio signals.
Geomagnetic storms (or periods of large and often rapid changes
in the intensity and direction of the various magnetic field com-
ponents) are almost always associated with ionospheric disturbances
which can have far-reaching impacts on global radio communications.
The daily SGDB reports provide data which can significantly enhance
the ability to determine the state of the ionosphere and when
November 13, 1991
- 2 -
conditions may begin to improve. They are also valuable for determin-
ing when conditions may become degraded.
There are a number of values contained in the daily SGDB reports
which are valuable to pay attention to when attempting to interpret
ionospheric activity. Each of the appropriate indices will be dis-
cussed below.
The 10.7 cm solar radio flux, the average 90-day solar flux, and
the sunspot number values are valuable for determining the overall
strength of the ionosphere. A stronger ionosphere will permit propa-
gation on higher frequencies. Rising values also indicate increased
levels of solar activity which may affect daytime propagation condi-
tions. To this end, the SWF (or Short Wave Fadeout) parameter is use-
ful for determining the estimated maximum number of episodes of
fadeouts of HF radio signals over the daylit hemisphere which may have
occurred during the last UT day. For example, if the SWF value read:
"SWF=05:079", you could interpret this to mean that there were possi-
bly as many as five episodes of HF signal degradation during the UT
day producing an estimated maximum of 79 minutes of signal degrada-
tion. It is important to remember that this figure only applies to
the daylit hemisphere. Sectors of the Earth which are in darkness
will not experience flare-related SWFs because the ionosphere is not
exposed to the flare x-rays which increase ionospheric absorption lev-
els primarily over the middle and low latitude regions.
The 90-day average solar flux value is an important parameter to
watch. It indicates the trend of solar activity and is also corre-
lated with the trend of ionospheric strength. Rising average values
often indicate a trend toward better ionospheric conditions, while
falling values are typically associated with a weakening ionosphere.
This value is also valuable for determining the approximate rate with
which the ionosphere recovers from disturbed conditions. High average
values may allow the ionosphere to recover more rapidly than would be
expected if the values were low. Although there are many other fac-
tors which must be taken into account, this is one value that might
occassionally be useful in this regard.
The background x-ray flux (BGND-XRAY) is as important as the 10.7
cm solar radio flux value. A high background x-ray flux (a value in
the C-class range - usually above C2.0 or C3.0) is often associated
with stronger propagation conditions. Maximum Usable Frequencies
(MUFs) are generally elevated, or tend to become elevated, when the
background x-ray flux remains consistently high for some time.
The average x-ray flux (XRAY-AVG) is useful for determining the
average "dose" of x-rays which the ionosphere intercepted during the
last UT day. Higher values may indicate elevated absorption levels
and/or elevated MUFs. This value is related to the background x-ray
flux as well as the frequency and magnitude of solar flaring. The
closer this value is to the background x-ray flux, the more stable
daytime propagation conditions are likely to be. The ideal situation
is to have a high background x-ray flux, zero SWF statistics, and an
x-ray average statistic which is very near to the background x-ray
November 13, 1991
- 3 -
flux value. Provided geomagnetic activity is also quiet, these condi-
tions are usually associated with strong, stable HF propagation condi-
tions.
Perhaps one of the most important indices which are given in the
daily SGDB reports are the geomagnetic A and K indices. Geomagnetic
indices for Boulder and estimated Planetary conditions are provided.
These values can be immensely useful for determining ionospheric con-
ditions and for discerning diurnal variations in activity which may
provide more stable openings on the HF bands. The Boulder indices are
commonly quoted because Boulder Colorado is a middle latitude site and
often accurately depicts activity through the middle latitude regions.
The estimated planetary indices are values which are obtained from
many geomagnetic observatories around the world, spatially distributed
in such a manner that an estimate of global activity may be obtained.
The estimated planetary indices given in the daily SGDB reports are
slightly biased towards the higher latitudes. Together, the Boulder
and Planetary indices provide an accurate picture of overall geomag-
netic activity.
The A-indices are computed using a linear scale. The K-indices
are based on an open-ended semi-logarithmic algorithm. Although a
discussion of the algorithms is beyond the scope of this document, the
interested reader may find it useful to look up these terms in the
document "GLOSSARY.DOC".
Generally, high A-indices are associated with degraded HF propa-
gation conditions. Values higher than approximately 15-20 may begin
to degrade polar and high latitude radio signals. Values in excess of
30 can degrade middle latitude signals, while values exceeding 40 can
degrade even low latitude signal paths. Geomagnetic A-indices higher
than 50 are associated with major geomagnetic storming and can have
profound impacts on the ionosphere. During such conditions, the polar
and high latitude ionosphere may be useless to use, while the middle
and low latitudes can often be strongly degraded. During most major
geomagnetic storms, the ionosphere produces significant instabilities
in the ionoshere which can cause such things as rapid flutter, fading,
odd reception of typically inaudible signals, and complete blackout
conditions. VHF propagation is often enhanced during these strongly
disturbed periods. Intense sporadic-E is often accompanied by auroral
activity and geomagnetic storms which can be sufficient to reflect or
scatter signals in the VHF and sometimes even the UHF bands. VHF
auroral backscatter is most often associated with high levels of
geomagnetic activity (ex. K-indices of 6 to 9, or A-indices greater
than 50 or 100).
The A-indices provide an overall picture of how disturbed
activity was during the last UT day. The K-indices break the UT day
into eight equal 3-hour segments. Activity during each 3-hour segment
is rated on a scale of 0 (very quiet) to 9 (very severe storm). Mag-
netic activity higher than 3 often degrades polar and high latitude
signal paths, while values in excess of 4 or 5 can begin degrading
middle latitude paths. Anything as high or higher than 6 can produce
significant ionospheric storming and HF signal degradation over all
November 13, 1991
- 4 -
latitudes.
To determine diurnal variations, examine the daily K-indices. If
you are interested in middle-latitude diurnal variations of activity,
consult the Boulder K-indices (BKI). If you are interested in the
overall estimated planetary activity, consult the Planetary K-indices
(PKI). Generally, stable communications can be established when the
midpoint of the signal path lies within the quietest zone of geomag-
netic activity. This zone is often in the daylit hemisphere of the
Earth. This may not always apply, particularly if the associated A-
indices are high (ex. above 30 or 40) and/or if signal propagation is
through the polar region.
For communicators who frequently transmit and/or receive signals
over or near the polar zone, it is valuable to pay attention to the
proton fluence values. Of particular importance in this regard is the
proton fluence at greater than 10 MeV (Million Electron Volts). This
statistic is identified in the daily SGDB reports as "FLU10". It is
simply the number of energetic particles detected at geosynchronous
satellite altitudes during the last UT day. If the daily report is a
summary of activity on 12 November, then the fluence value is a valid
summary of proton activity for the UT day of 12 November.
The reason proton fluence at greater than 10 MeV is important is
due to the behavior of protons at these energy levels. Solar protons
at greater than 10 MeV are often ejected from major solar flares.
When these protons reach the Earth several hours after the major
flare, they are redirected by the Earth's geomagnetic field to the
polar regions where they penetrate into the ionosphere. As they
penetrate, they collide with constituents of the ionosphere and
thereby ionize it. The ionization can become quite intense during the
stronger events. This ionization may absorb radio signals propagated
from, through, or near the polar regions. This Polar Cap Absorption
(or PCA) can completely blackout radio signal transmission and recep-
tion from the polar and sometimes high latitude regions. Therefore,
persons who attempt to communication over or near the polar regions
during times when the proton fluence at greater than 10 MeV is high
may not be able to get the signals through. A background (or low)
proton fluence level at greater than 10 MeV is approximately associ-
ated with values less than 9.0E+03. Fluence values near or below this
value will not degrade polar path signals through absorption. If
polar path signals are degraded when fluence levels are low, the cause
is almost certainly geomagnetic (and aurora) related.
In order to determine the effect of greater than 10 MeV protons
on polar path propagation, examine the PCA statistics in the daily
SGDB reports. When the proton fluence at greater than 10 MeV is high,
PCA levels will also be elevated. Small changes in the average PCA
levels (PCA-AVG) can have profound impacts on radio communications
over the polar regions, when the PCA statistics are near or above
approximately 0.4 or 0.5 dB. PCA values higher than 1.0 dB are usu-
ally associated with strong absorption of HF radio signals over the
polar regions. PCA values in excess of 2.0 dB can cause complete
blackout of signals over the polar regions. During very strong events
November 13, 1991
- 5 -
(ex. where PCA levels may reach or exceed 10.0 dB), even the high
latitude and some northerly middle latitude regions can be completely
blacked out, making all ionospherically propagated radio communica-
tions over these regions futile. PCA values are generally only appli-
cable when the proton fluence at greater than 10 MeV is elevated.
Proton fluence values at greater than 1 MeV are less critical to
polar path signal propagation, but still may have a small affect.
Generally, the proton fluence at greater than 1 MeV must be much
higher than normal in order to affect polar path signals (provided the
fluence at greater than 10 MeV is also low). Experience in observing
these indices will show what levels of fluence can affect propagation
of radio signals through or near the polar regions.
One other important parameter to watch is the neutron monitor
statistics (NEUTN-MAX, NEUTN-MIN, NEUTN-AVG). Many ionospherically
influential geomagnetic disturbances affect the neutron monitor counts
as observed at Thule, Greenland. The trend of the neutron data can be
useful for estimating the state of the ionosphere. Generally, a
strongly degraded ionosphere is associated with negative neutron moni-
tor statistics (ex. a NEUTN-AVG value below -2 to -5 percent). A nor-
mal ionosphere is usually associated with values near or above zero
percent. Positive neutron statistics should not be interpreted to
mean ionospheric conditions are better than normal. The correlations
are not perfect, but can be indirectly related.
During strong geomagnetic storms, the neutron counts may fall as
low as -5 or -10 percent. In almost all cases where strong negative
drops in neutron counts are observed, the recovery phase is relatively
slow. A full recovery back to near zero percent may take several days
to a week. The time required for the neutron counts to recover is
often similar to the time required for the ionosphere to recover to
near normal. These statistics are therefore often valuable indicators
for determining the approximate recovery state of the ionosphere fol-
lowing strong storm events.
Since geomagnetic activity which is influenced by recurrent solar
coronal holes is often recurrent itself, the statistics listing the A
and K-indices which were observed 26 and 27 days ago are valuable for
estimating the possible impact of recurrent activity on the geomag-
netic field. These statistics may be found in the daily SGDB reports
equated to the "27DAY-AP" and "27DAY-KP" acronyms. These values can
be used as guides together with the forecast K and A indices to deter-
mine the possible impact of future recurrent activity on the geomag-
netic field and the ionosphere.
The A-indices are forecasted 72-hours into the future by the
Space Environment Service Center in Boulder Colorado, and by the
United States Air Force (providing the forecasted planetary A-
indices). The statistic "BAI/PAI" represents the 72-hour Boulder A-
indices forecast and the 72-hour Planetary A-indices forecast respec-
tively. The "KFCST" values provide estimated forecast K-indices for
each three-hour period of the next 48 hours.
November 13, 1991
- 6 -
By inspecting and studying the indices mentioned above and their
trends, persons interested in communications can much more effectively
determine when conditions might be favorable for stable HF signal pro-
pagation. To this end, the PC-compatible software program "BCAST"
will prove to be an invaluable tool to those who want an advantage in
identifying when and where to find the best conditions for reliable HF
DX.
3. Determining When to Watch for Aurorae
The most important parameters of the daily SGDB reports which can
be used to watch for auroral activity are discussed below. Observing
auroral activity is not as unpredictable as many believe. By monitor-
ing the data contained in the daily SGDB reports, people interested in
observing aurorae can become quite successful aurora observers. To
better understand the latitudinal boundaries associated with the fol-
lowing discussion, consult the document "GLOSSARY.DOC".
Since auroral activity is highly correlated with geomagnetic
activity, one of the most valuable indices that can be monitored are
the geomagnetic activity indices and forecasts. The Boulder and
Planetary K-indices (BKI and PKI respectively) are important indica-
tors of auroral activity. The higher the indices are, the more visi-
ble auroral activity is likely to be. K-indices in excess of 4 can
produce good displays of aurorae over the northerly middle and high
latitude regions. Values of 6 or more are usually associated with
geomagnetic and auroral storms. During these events, activity can
often be seen well into the middle latitude regions. Values of 8 or
more are well correlated with observations of auroral activity well
into the low latitude regions. Consistent K-indices of 9 may occas-
sionally produce observations of aurorae into the tropics.
Similar to the K-index, A-indices can also be used to determine
the approximate intensity and visibility of auroral activity. A-
indices in excess of 25 produce displays of aurorae over the high and
northerly middle latitudes. As values increase, the auroral zone (ex.
the area where aurorae is seen directly overhead) migrates equator-
ward. During intense geomagnetic and auroral storms, the auroral zone
can lie well into the middle latitude regions. The auroral ovals are
typically isolated to the high latitude regions in Northern Canada and
south of Australia (one over each pole). A-indices in excess of 40
can produce very impressive auroral displays over the northerly middle
and high latitude regions. Values in excess of 100 are associated
with observations from low latitude regions. Activity when A-indices
are 100 or more is most intense over the middle latitude regions where
the auroral oval has migrated equatorward. During the rogue storms,
where A-indices surpass 200, tropical observations of activity may
become possible.
In order to determine the possible visibility of future activity,
you will need to determine the forecast K or A-indices from the daily
SGDB reports and apply them as indicated above. You will also need to
know the phase of the moon, since the luminosity of the moon can have
a profound influence on the visibility of auroral activity. For the
November 13, 1991
- 7 -
latter, simply look out the window. Another important parameter is
the background brightness of the sky. Light pollution caused by
cities or nearby towns can degrade or completely obscure the light
from aurorae. For the best views of activity, a dark sky site in the
country and away from bright city or town lights is required.
Optimal conditions occur when the forecast K or A-indices are
high, the lunar phase is such that the moon does not rise during the
night (or remains below the horizon for several hours after evening
twilight), and the sky is clear and stable.
In most cases, recurrent geomagnetic and auroral activity pro-
duced by solar coronal holes are less influential than events gen-
erated by major solar flares or long-duration minor flares. However,
there are occassional exceptions to this. In general, the overall
intensity of recurrent activity can be estimated by examining the
forecast A and K indices as well as the activity which occurred 26-27
days ago, using the "27DAY-AP" and "27DAY-KP" fields in the daily SGDB
reports.
In addition to the above indices, it can be worth-while to pay
attention to the warning section of the daily SGDB reports. There are
four warnings which are explicitly devoted to auroral activity. These
are: AURMIDWCH (Middle Latitude Auroral Activity Watch), AURMIDWRN
(Middle Latitude Auroral Activity WARNING), AURLOWWCH (Low Latitude
Auroral Activity Watch) and, AURLOWWRN (Low Latitude Auroral Activity
WARNING). The watches are generally issued when activity might become
visible. Warnings are released either when activity has been con-
firmed over the stated latitude region, or if there is a strong proba-
bility that activity may be observed in the future.
When the watches and warnings are used together with the fore-
casted and observed A and K-indices, persons interested in observing
aurorae are much more liable to witness activity than those who are
not so well equipped. Studying the trend of geomagnetic activity (for
example, using the BCAST software program) can also be a significant
aid in determining when the next episode(s) of auroral activity may be
observed.
November 13, 1991