During a total solar eclipse, day becomes night for a few precious minutes. The temperature drops, birds stop singing, and bees return to their hives for a premature rest. An eerie quiet envelopes the landscape inside the path of totality.
However, from the unique perspective of a ham radio operator, night is anything but quiet - and neither is a solar eclipse. Many shortwave radio stations that are undetectable in daytime are easy to pick up at night. The reason has to do with the Sun's effect on Earth's atmosphere. During the day, solar UV radiation ionizes atoms and molecules in the upper atmosphere, creating a zone called the "ionosphere." The uppermost part of the ionosphere, called the F layer, is so thoroughly ionized that some free electrons exist there - even at night - when the UV source (the Sun) is not present. The F layer is like a mirror for radio waves with frequencies below about 20 MHz. Shortwave transmissions from earth hit the F layer and bounce back down. In fact, many such bounces can occur, and this is the reason why over-the-horizon transmissions are possible at short wave frequencies.
Interestingly, the ionosphere -- so important for long distance radio communication -- can also dampen radio waves. In the D and E layers the degree of ionization is not as great as in the F layer. Partial ionization cause these layers to act more like a resistor than a mirror. Short waves passing through them are attenuated rather than reflected. Signals at some frequencies are damped out altogether. Fortunately, the level of ionization in the D layer is small, and when the sun sets at night, all the molecules and free electrons can recombine, and the D layer disappears! Stations that were damped out during the day can then propagate around the world.
This well-known atmospheric ritual takes place every day. On August 11, 1999 it will happen twice.
As the path of totality slices through Earth's atmosphere, ions and electrons in the vicinity of the shadow will begin to recombine. The reflecting F layer may not be greatly affected, but ionization in the attenuating D layer could vanish. Shortwave radio stations that were restricted in range to sites in Europe just moments earlier may be able to skip over the horizon and be heard on the other side of the Atlantic.
Solar Disk Jockeys Scientists at NASA/Marshall are putting this phenomenon to the test by inviting Science@NASA readers to become "Solar Disk Jockeys," who will report the effects of the August 11th solar eclipse not by watching for it, but by listening for it. Since England and middle Europe offer unpredictable visual conditions, the audio eclipse may prove the most reliable observation, particularly when heard from thousands of miles away.
The BBC World Service will be a good choice for many radio listeners. BBC transmitters are located mostly in the UK relatively near the path of totality, and they transmit at frequencies between 5 and 15 MHz that are favorable for probing changes in the D-layer. The table, below, shows BBC frequencies and suggested monitoring times. European Voice of America transmitters may also be suitable. A list of VOA frequencies for Europe is given at http://www.voa.gov/europe.html.
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Interested readers should use their shortwave receivers to experiment with different stations. The best ones for eclipse listening will be transmitters that can be heard at night, but not at all during the day. The eclipse will begin over Europe around 10:10 UT when it is still night over most of North America. Thus, when the daytime ionosphere begins to diminish over Europe, there is a good chance that European shortwave broadcast stations will be able to propagate great distances into the Western Hemisphere.
Recording useful data is easy: Simply note the following information: The station's signal strength as displayed on the receiver's VU meter at night and during the local time of the eclipse for a week centered on Aug 11. Then email your logs, your audio recordings (if any), and your position (latitude and longitude) to Marshall Space Flight Center's Eclipse mailbox. The data will be analyzed to help determine ionospheric properties, and used for a future story from Science@NASA.
"A total eclipse of Sun is about as close to a controlled experiment as atmospheric research can hope for." Marcos A., Penaloza M.,University of Essex, Institute for Environmental Research
The possibility of "listening" to the August 1999 eclipse from afar has more than novelty value. Scientists are interested, too, because the results of a global monitoring experiment could give them new insight into the physics of the upper atmosphere.
Radio Heaven
In the last 1000 years, the 25th most significant human event was the first over-the-horizon radio transmission, according to Life Magazine's millennium edition--a 2,000 mile space broadcast of the Morse code letter, "S", carried from a kite in Italy to a receiver in New York. In 1999, there are 4 million amateur radio operators in the US alone, and they continue to increase around 7% per year.
As the Moon's shadow moves across the earth the temperature in the upper atmosphere will drop, changing the wind pattern as air contracts in on the eclipse region. In the absence of UV radiation from the Sun, the ionosphere rapidly begins to decay. The shadow of the moon races through the atmosphere at supersonic speeds, causing wind and waves of electron-ion recombination to spread through the atmosphere from the eclipse region. The effects could be global.
All of these phenomena have been predicted by theory, and some have been observed during previous eclipses. On August 11, European scientists and amateur radio operators will attempt to refine physical models of the ionosphere by attempting to monitor changes in the D layer absorption of radio beacons. The public is invited to participate.
Theoretical physics will benefit from an improved understanding of the ionosphere, and so will radio broadcasters. In many atmospheric models the attenuation of the D layer is overestimated, with the result that radio transmitters are often operated at a higher power than necessary. This, in turn, costs money, wastes energy, and pollutes the already cluttered airwaves with more RFI (radio frequency interference). Improved models of radio absorption in the ionosphere could lead to lower power broadcasts during much of the day.
"With the measurements of absorption obtained during the eclipse, scientists and engineers hope to have a better understanding of the nature of the ionospheric absorption which may ultimately lead to less interference in the future." (quote from Rutherford-Appleton Lab, UK.)
Calling All Solar Disk Jockeys
For sky watchers in North America and other areas not touched by the path of totality, August 11, 1999 offers a unique opportunity to sense the eclipse from a distance (or perhaps from beneath a cloud if you live in Britain). Science@NASA encourages readers to tune into the eclipse using shortwave radios and to report their results for scientific analysis. Reader suggestions concerning appropriate frequencies and observing procedures are welcomed and will be distributed to other "Solar Disk Jockeys" prior to totality.
Science@NASA will need the following information for a meaningful record of observations.
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a) the frequency of the radio station(s),
b) the station's signal strength(s) as displayed on the receiver's VU meter at night and during the local time of the eclipse for a week centered on Aug 11, c) your location (latitude and longitude).
Send these logs and your audio recordings (if any) to Marshall Space Flight Center's Eclipse mailbox (eclipse@msfc.nasa.gov). The data will be analyzed to help determine ionospheric properties, and used for a future story from Science@NASA.
For more information about the August 11, 1999 solar eclipse, please visit Goddard Space Flight Center's Solar Eclipse home page.