N2LVI's Quick Guide to HF Propagation Using Solar Indices

Graphics:  Auroral Activity  |  Solar X-Ray Flux  |  Sunspots  |  SOHO  |  D-Region Asorbtion  |  MUF  |  Solar X-Ray Absorption  |  E-Layer

Current Index Numbers from WWV



While increasing SFI may be good for HF propagation, it also tends to correspond with high Ap and K indices, which cause D-Layer absorption and noisy band condition.  Solar flux is measured in solar flux units (SFUs).  It is the amount of radio noise or flux emitted at a frequency of 2800 MHz (10.7 cm, hence is it also called the 10.7 cm flux index).

NOAA broadcasts geophysical alert messages over WWV at 18 minutes after the hour and from WWVH at 45 minutes after the hour. The messages are less than 45 seconds in length and are updated every 3 hours (typically at 0000, 0300, 0600, 0900, 1200, 1500, 1800, and 2100 UTC).



The subscript "p" means planetary and designates a global magnetic activity index.  The following 13 observatories, which lie between 46 and 63 degrees north and south geomagnetic latitude, now contribute to the planetary indices:  Lerwick (UK), Eskdalemuir (UK), Hartland (UK), Ottawa (Canada), Fredericksburg (USA), Meannook (Canada), Sitka (USA), Eyrewell (New Zealand), Canberra (Australia), Lovo (Sweden), Brorfelde (Denmark), Wingst (Germany), and Niemegk (Germany).



K indices isolate solar particle effects on the earth's magnetic field;  over a 3-hour period, they classify into disturbance levels the range of variation of the more unsettled horizontal field component.  Each activity level relates almost logarithmically to its corresponding disturbance amplitude.  Three-hour indices discriminate conservatively between true magnetic field perturbations and the quiet-day variations produced by ionospheric currents.  K indices range in 28 steps from 0 (quiet) to 9 (greatly disturbed) with fractional parts expressed in thirds of a unit.  A K-value equal to 27, for example, means 2 and 2/3 or 3-;  a K-value equal to 30 means 3 and 0/3 or 3 exactly;  and a K-value equal to 33 means 3 and 1/3 or 3+.  The arithmetic mean of the K values scaled at the 13 observatories listed above gives Kp.



The a-index ranges from 0 to 400 and represents a K-value converted to a linear scale in gammas (nanoTeslas, nT) a scale that measures equivalent disturbance amplitude of a station at which K=9 has a lower limit of 400 gammas.

 K 0 1 2 3 4 5 6 7 8 9
 a 0 3 7 15 27 48 80 140 240 400


Solar X-rays: 
Geomagnetic Field: 

Auroral Activity (North)

Click on map for NOAA website

Auroral Activity (South)

Click on map for NOAA website

Solar X-Ray Flux

Click on chart to see current chart


Click on image for NASA Sunspot website

SOHO Solar Images

Click on Image for NASA SOHO Images

D-Region Absorption

Click on map for NOAA website


Maximum Useable
Frequency (MUF)

Maximum Useable Frequency (MUF)

Solar Terrestrial Dispatch

20 MHz Solar X-Ray

20 MHz Solar X-Ray Absorption

Solar Terrestrial Dispatch

E-Layer Critical
Frequency Map

E-Layer Critical Frequency Map

Solar Terrestrial Dispatch


Additional Web References

http://www2.arrl.org/tis/info/pdf/0209038.pdf     ARRL - Understanding Solar Indices, by Ian Poole, G3YWX

http://www.spaceweather.com     Daily space weather updates

http://www.dxlc.com/solar     Solar Terrestrial Activity Report

http://crlgin.nict.go.jp/sedoss/solact3     Solar-Terrestrial Activity Chart (Japan)

http://www.sec.noaa.gov/ftpdir/indices/DGD.txt     Recent K and A values

http://sidc.oma.be     Space Weather: The International Journal of Research and Applications (Belgium)

http://solarscience.msfc.nasa.gov/SunspotCycle.shtml     The Sunspot Cycle

http://www.spaceweather.com/glossary/sunspotnumber.html     Sunspot numbers and 400-year chart

http://www.eham.net/DX/propagation     Links to many useful websites, with relevance ratings

http://www.ngdc.noaa.gov/stp/GLOSSARY/glossary.html     Glossary

http://www.arrl.org/tis/info/k9la-prop.html     ARRL tutorial on solar effects on propagation

http://www.sunspot.noao.edu/PR/answerbook/sunspots.html     Sunspot answer book

http://www.sec.noaa.gov/radio     NOAA's Space Environment Center Radio User's Data

http://soho.nascom.nasa.gov     Current SOHO images

http://www.dxinfocentre.com/tropo.html     William Hepburn's Tropospheric Ducting Forecast

http://www.arrl.org/w1aw/prop     ARRL Propagation Bulletins

Additional VARA/W2VTM References

Working the Gray Line Tutorial - by Paul Hardin, NA5N

The Handyman's Guide To Solar/Geomagnetic Conditions - by Paul Hardin, NA5N

Space Communications Education/Reference Page - Links to space-related websites


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Contact:  Peter Greene, N2LVI
Last Updated:  25 July 2012

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