When a meteor enters the atmosphere, it will leave an ionized trail at an altitude between 80 km and
120 km.  These ionized trails are capable of reflecting radio signals. By looking for these reflections
the meteor can be observed. The advantage of this kind of observation is that they can be made
regardless of the conditions. Observations can be done with a full moon, in daylight and during cloudy
or rainy weather.  Radio detection also gives higher observation rates than visual observation rates.
The reason this occurs is because the ionized track is larger than the meteor itself. 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 diameter of about 3 mm visually and
0.06 mm by radio, respectively. By using radio observation you can detect meteors down to 8th
magnitude!

The way in which a meteor trail reflects radio waves depends on the density of free electrons in the
trail. Generally, there are two types of meteor trails. Under dense, the ones with very low electron
densities, less that 2 x 10^14 electrons per meter, and over dense, the ones with very high electron
densities, over 2 x 10^14 electrons per meter. Under dense meteors, of course, correspond to faint
meteors. Observations of under dense reflections can detect a meteoroid mass range that is not
observable with the naked eye.

The  profile of the meteor is a graph of the received signal strengths. Under dense and over dense
meteor trails have different profiles.  The profile of an under dense trail (fig. 1) shows a very steep rise
of a few hundreds of a second which is followed by an exponential decay . The signal last no more
than a few tenths of a second.  Over dense trails (fig. 2) can last for seconds with the signal strength
unpredictable and often highly irregular.
Meteor Observation
Apart from the simplicity of the received signal strength profile, under dense meteors have two other
qualities. They are very numerous (allowing the use of statistical techniques) and they strictly obey
the specularity rules. This property creates a link between the position of the meteor reflection point
in the sky and the meteor path in the atmosphere. This is especially useful with the radio observation
of meteor streams.
Overdense meteors are less numerous and correspond to brighter meteors. Because the trail is
large, it is often spread about by the wind. This is why the profile is often very irregular. Because of
the spreading of the trail by the wind, there is not always a link between the reflection point and the
location of the meteor path in the atmosphere.  Statistical  reduction of over dense meteor
observations is therefore difficult.
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fig. 1
fig. 2
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