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

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                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



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                                   - 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



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   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



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   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



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   (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.




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        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



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   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