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Radio Detection of Meteors
By Philip Gebhardt
The term "meteor" applies not only to the streak of light produced by a meteoroid, but also to the column of ionized atoms and molecules along the path behind the meteoroid. These meteor trails are capable of scattering radio signals from terrestrial stations. Unlike visual observation, radio detection of sporadic meteors or shower meteors can be undertaken in daylight and during inclement weather. Similarly, a night sky illuminated by the full Moon has no adverse effect on radio detection. Radio detection rates tend to be higher than visual observation rates because particles down to 10-5 kg can be detected visually, while particles down to 10-10 kg can be detected by radio. Assuming a density of 1 t/m3, these mass limits correspond to diameters of about 3 mm and 0.06 mm, respectively.
Two types of meteor trails exist, underdense and overdense; they are determined by the density of free electrons. Reradiated signals from underdense trails (fewer than 2 x 1014 electrons per metre) rise above the receiver noise almost instantaneously and then decay exponentially. The duration of many meteor bursts is about a second or less. Reflected signals from overdense trails may have higher amplitude and longer duration, but destructive interference due to reflection from different parts of the trail can produce fluctuations in the signal. Note that other means of signal propagation may be heard on the FM band, but only meteor signals have their characteristic fast rise-time and short duration.
Data for selected meteor showers appear in the table at the left. These data are for visual observations and should be considered as guidelines only for radio purposes. The sporadic meteor rate (visual and radio) peaks about 6 a.m. local time (i.e. on the advancing side of Earth in its orbit) and is minimum near 6 p.m. The rate will vary from a few per hour for off-peak times for sporadic meteors to several hundred per hour during a very active shower. Frequencies between 20 and 150 MHz are typically used for meteor detection. Both amplitude and duration of meteor bursts are frequency-dependent-they decrease with increasing frequency. At the lower frequencies, however, galactic as well as human-made noise (particularly in urban areas) become limiting factors. Also, as the wavelength becomes comparable to the width of the meteor trail, the echo strength decreases.
The commercial FM broadcast band (88 to 108 MHz) provides the best introductory opportunity for meteor detection. The abundance of over-the-horizon stations transmitting 24 hours a day ensures that a meteor burst can be heard from a suitably positioned meteor regardless of the time of day.
The technique involves listening on a frequency not used by a local FM station. A receiver with a digital frequency readout is therefore an asset. Frequencies throughout North America are assigned at 200-kHz intervals between 88.1 and 107.9 MHz. In the absence of a clear frequency, it is possible to use a frequency occupied by a station with a very weak signal although weak meteor bursts will be masked. Alternatively, a TV set (channels 2 through 6) can be used, provided the set is connected to an antenna rather than through cable TV. For either the FM band or the TV channels, an outdoor antenna is preferable. It is also possible to listen using the FM radio in your car.
Further information can be found on the following websites:
Also see Sky & Telescope, 94, no. 6 (December 1997), p. 108.
Excerpted from the Observer's Handbook 2002 © The Royal Astronomical Society of Canada, 2001. Used with permission of The Royal Astronomical Society of Canada. The Observer's Handbook can be ordered at http://www.rasc.ca/publications.htm.