Audio Eclipse May Fill the Sky
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Audio Eclipse May Fill the Sky
Hams around the world can track the August 11, 1999
total solar eclipse by monitoring changes in atmospheric radio
propagation.
Aug 7, 1999: UPDATE - A special transmission of the BBC World Service will be broadcast from Rampisham, England near the path of totality to assist NASA efforts to monitor atmospheric radio propagation conditions during the eclipse. [details]
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 reduce the energy in 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 cycle 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.
| Time (UT), August 11 | Frequency(s) kHz | Site(s) | Target of Broadcast |
|---|---|---|---|
| 7325, 15190 | Rampisham (special broadcast), Ascension | N America, S America | |
| 10-1100 | 5965, 6195, 7325 | Sackville, Antigua, Rampisham (special broadcast) | N America, C N America, N America |
| 11-1200 | 5965, 15220, 15190, 6195, 5975 | Sackville, Antigua, Ascension, Cyprus, Greenville | N America, Americas&Carib, S America, C N America (begins 11:30 UT), C S America |
| 12-1300 | 9515, 6195, 15220 | Sackville, Cyprus, Antigua | N America, C N America (begins 12:16 UT), N C S America (begins 12:16 UT) |
| 13-1400 | 15220, 9515, 6195 | Antigua, Sackville, Cyprus | N C S America, N America, C N America |
Above: This table lists some short-wave frequencies for
the British
Broadcasting Corporation (BBC) world service. The table suggests
the times (in Universal
Time) when hams may detect the clearest transmission during
the eclipse event. Note the target of transmission - we suggest
selecting a target closest to home. European Voice of America
transmitters may also be interesting. Ham operators should
experiment before August 11 to select frequencies to log during
the eclipse. Click for a more
complete table showing times and targets of BBC shortwave broadcasts
(including worldwide targets) during the eclipse. (Source of
information for tables: Dan Ferguson, Spectrum Management Division,
International Broadcasting Bureau)
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
S 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.
Special Live Audio Webcast
Thanks to the assistance of the International Broadcasting Bureau, the UK's Merlin Communications is joining forces with Science@NASA for a special broadcast of the BBC World Service to probe atmospheric conditions during the eclipse. The transmitter is located at Rampisham in southern England (50o 48' N, 2o 38' W), not far from path of totality. The 500 kW signal will be broadcast at 7325 KHz over a 4x4 curtain antenna with a 30 deg horizontal beam width. The antenna will be pointing toward 285o azimuth, approximately in the direction of Washington DC as viewed from Rampisham.
Transmission times:
- regular evening show at 2:45 to 3:45 UTC (10:45 p.m. to 11:45 p.m. Central Daylight) braodcast daily
- special broadcast: August 10 0900-1100 UTC (4 a.m. to 6 a.m. CDT) the day before the eclipse
- special broadcast: August 11 0900-1100 UTC (4 a.m. to 6 a.m. CDT) during the eclipse!
Right: A map of the expected changes in radio
MUF (maximum usable frequency) caused by the August 1999 solar
eclipse. The striking aspect of these University College London
simulations is that the changes are global, not local to the
path of totality. With times shown at top, the numerical simulation,
performed by University College, London, will be used to support
a collaborative experiment between UCL and the Rutherford-Appleton
Lab, in which radio signals between Spain and England will probe
the eclipse on selected MW frequency and radios tuned by the
public. [more
information from Rutherford-Appleton Labs] "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.
Educators! Thursday's Classroom for lesson plans and activities related to the August 11, 1999 total solar eclipse. |
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 (see "related links" below).
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.
- 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. 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.
Radio
frequencies reference - Voice of America's
European stations Rutherford-Appleton Laboratory (UK) research on eclipse effects:
Further reading: [1] Journal of Atmospheric and Terrestrial Physics Volume: 38 Page: May 1976 Meisel, D. D. (New York, State University) Duke, B. (Canadian Broadcasting Corp.) Aguglia, R. C. (Buffalo Museum of Science) Goldblatt, N. R. (Rochester Institute of Technology) p. 8 [2] Kolokolov, L. E. Legen'ka, A. D. Pulinets, S. A. (IZMIRAN) Geomagnetizm i Aeronomiia Volume: 33 Issue: 1 Page: p. 49-57. Feb 01, 1993 [3] Zaitseva, N. A. Elanskii, N. F. Matveev, A. D. Vartanian, V. A.Meteorologiia i Gidrologiia Page: 45-50 Jun 01, 1984 More Space Science Headlines - NASA research on the web NASA's Office of Space Sciencepress releases and other news related to NASA and astrophysics |
Science Notes from Previous Eclipses A multifrequency sounding experiment bounced high-power radio
waves off the Earth's atmosphere during the July 10, 1972 eclipse,
and a research team led by New York State University and the
Canadian Broadcasting Corporation found: "A definite correlation
of very low frequency (VLF) phase, high frequency (HF) signal
strength, and geomagnetic field behavior on eclipse day."
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