Website: Walter Gregg (Walt.Gregg.Juneau.AK.US)

[Auroral Kilometric Radiation (16 sec midi sound)]

 Solar Terrestrial Conditions

The current solar flare level is indicated as [X-Ray Flux] .[d] [See (formerly Majestic Research).] The active solar region may show sunspot groups or flares. An image taken in the orangish yellowish glow of the hydrogen alpha wavelength at 6562.8 angstroms darkens the sun sufficiently to show flare activity, which is generally invisible in white light. Solar flare activity is of interest because it is strongly correlated with maximum observed shortwave frequencies. It's believed that solar X-Rays and ultraviolet light increase ionization in earth's ionosphere, thereby boosting maximum observed frequencies. However, this also increases absorbtion in the lower 'D' region of the ionosphere. Strong flares can result in a shortwave radio fadeout within a minute and lasting up to several hours. You may want to view NOAA's current D-layer absorbtion map and solar X-Ray graph. The underlying GOES 10 solar X-Ray data is also available.

The current planetary geomagnetic field disturbance indicator is indicated as the Kp index .[d] [See (formerly Majestic Research).] Disturbances in the geomagnetic field indicate corresponding ionospheric disturbances such as ionospheric storms. These storms are violent disturbances of the entire ionosphere. They begin in the auroral zones, where they destroy the normal stratification of the ionosphere into the refracting E, F1, and F2 layers upon which shortwave depends. Within hours the effects spread to lower latitudes. Maximum usable frequencies plummet and absorbtion increases. Severe storms can cause a complete shortwave radio blackout for up to several days. These storms only occur after solar flares, normally delayed by 17 hours to five days. However, most flares fail to result in even a mild storm. Storms occur only if a flare happens to emit particle streams toward earth. Such streams drive the aurora, so ionsopheric storms are accompanied by auroral activity. You may want to view NOAA's current map of auroral activity, which now includes an auroral activity index. This index may be a better indicator of shortwave propagation effects than the Kp index is. You may also want to view NOAA's current Kp index bar chart.

Running annual averages of sunspot activity and solar radio noise correlate very well with solar flare activity and maximum usable frequencies, but there is very little daily correlation. Solar activity is traditionally stated as the running average of the daily Wolf sunspot number R. This is because sunspots can be clearly seen in white light while only the brightest portions of major flares can be seen in such images. The running annual average Wolf number correlates very well with overall flare activity and thus maximum usable frequencies. More recently, running running averages of the 10.7 centimeter (2800 megahertz) solar radio noise flux F measured at local noon have been used as a more convenient proxy for solar flare activity. It is possible to convert between long term averages of the 10.7cm flux and the Wolf sunspot number using the following formulas:

F = 67.0 + 0.572R + (0.0575R)² - (0.0209R)³
R = 1.61(F - 67.0) - [0.0733(F - 67.0)]² + [0.0240(F - 67.0)]³

[The sunspot number may be estimated from whole sun images (138k jpg).] [d] The Wolf sunspot number R is the total number of sunspots plus 10 times the number of sunspot groups, all multiplied by an observatory factor. It takes a practiced eye and clear weather to obtain consistent results from such observations. A running average of the 10.7 cm solar flux F is now commonly used in place of sunspot observation. (Recent solar image from Big Bear Observatory.)

Cosmic Ray Activity. It is thought that only the strongest solar flares generate cosmic rays, but there is always a galactic background, constituting some 15% of the total radiation exposure at the earth's service. The intensity is up to 100 times higher at commercial aircraft altitudes. Cosmic ray activity is estimated by measuring neutrons. There are dozens of research stations publishing this information. The Moscow station includes an up to the minute graph.


Natural sources of electromagnetic signals such as lightning and electrical storms are best known for producing broadband radio static, also called atmospherics. The short term, spherics, is understood to encompass a wide variety of spheric sounds (44 sec sound m3u playlist of mp3 clips) that can be heard by simply connecting an audio amplifier and speaker to an appropriate antenna in an area removed from power lines. Listening to spherics makes an interesting hobby and monitoring them has some use in studying the ionosphere as well.

Solar Flare and Magnetic Storm Monitoring. The continuous static from thunderstorms thousands of miles away peaks at around 27 kilohertz. The strength is normally about 15 decibels less during daylight than darkness. But solar flares often cause what is known as a sudden enhancement of atmospherics (SEA): the daytime strength suddenly leaps to night levels. The effect is relatively easy to measure and is one method of detecting flares. On the other hand, a magnetic storm has little effect on daytime atmospherics (it may rise 3 decibels) but nighttime atmospherics may fall some 15 decibels. Other frequencies are also affected but the results may be very different at as little as half or double the 27 kilohertz frequency. Monitoring at 27 kilohertz as used by the sudden ionospheric disturbance (SID) program, solar division, American Association of Variable Star Observers (AAVSO) ( is an inexpensive way to gain some insight into solar-terrestrial conditions. Receiving systems were described in the Amateur Scientist of Scientific American for September 1960 and in the June 1973 issue of Sky and Telescope. More recent information can be obtained through the AAVSO.

Sound Effects. The aforementioned sounds of sferics (repeated as individual clips here) include whistlers, dawn chorus, and auroral kilometric radiation, a sound that can only be heard from space. One of the reasons the sound effects occur is that the wideband static may travel along the lines of the earth's magnetic field for many earth radii away before returning to earth, and the higher frequencies travel faster along these paths. The classic result is is heard as a falling tone known as a whistler (11 sec mp3 sound). The dawn chorus (15 sec mp3 sound) variant, short rising tones synchronized with auroral bursts, occurs when electrons spiral down into the atmosphere. You may have heard a representation of Auroral Kilometric Radiation (16 sec midi sound) when you opened this page. This was produced by Stepan Andreenko's Music Recognition System (V2.0 for os/2 and win9x 1998)( from a NASA satellite recording of Auroral Kilometric Radiation (18 sec mp3 sound), which actually sounds like birds chirping. To read more about what causes such signals or hear recordings in Apple AIFF or Windows WAV formats, visit Sounds of the Magnetosphere. For truly fantastic sound clips, visit Auroral Chorus, Stephen P. McGreevy's site, where you can hear over a dozen MP3 samples, order a CD-ROM of recordings, or obtain information on making your own.

Credits and Additional References

  1. Icons summarizing Solar X-Ray Flux and Geomagnetic Field disturbance are provided by n3kl (, with links to additional solar images and data.
  2. Formulas for converting between sunspot number and solar flux are from Richard Thompson, The Ten Centimetre Solar Radio Flux (May 3, 1995)(IPS Radio & Space Services, Syndey, Australia; at /papers/richard/conversions.html; now via
  3. Sounds are from NASA: Sounds of the Magnetosphere ( at /istp/polar/polar_pwi_sounds.html).
  4. Except where noted or at external sites, other images, graphs, and data are provided by the NOAA Space Weather Prediction Center (
  5. Perhaps the best description of the ionosphere's effect on shortwave radio is still found in Frederick Emmons Terman's Radio Engineers' Handbook pp. 709-33 (1943)(McGraw-Hill) in the section titled "The Ionosphere and its Effect on the Propagation of Radio Waves," particularly under "Abnormal Ionosphere Behavior" at pp. 723-25.
  6. Limited additional information about SEA and SID monitoring may be found in Dave Heiserman's Radio Astronomy for the Amateur at 217-18 (1975)(Tab Books #714, ISBN 0-8306-4714-7 (paperback) and 0-8306-5714-2 (hardback)).

, Solar Terrestrial Conditions (2002 Rev. 2008).
Subjects: 1.  Atmospheric Physics -- Earth Sciences. 2. Sun -- Solar System.
( 2002/ text/ 2002a00-solar-storms).