| August 4, 1999: 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 JockeysScientists 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.
Left: This table
lists short-wave frequencies for the British Broadcasting Corporation
(BBC) world service,
whose transmitters are located relatively near the path
of totality. The table suggests the times (in Universal Time) when hams may detect the clearest
transmission during the eclipse event. European Voice of America transmitters may also be interesting. Ham
operators should experiment before August 11 to select
frequencies to log during the eclipse. 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." 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.
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.
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.
Send these logs and your audio recordings (if any) 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.
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