Reflection of radio waves from the meteor trail

I was very interested in the following article and asked its author – Ladislava Bálinta – for consent to its publication on CQ.sk. Ladislav's page “Meteory” she is great, so don't miss it. Her address is: http://meteory.persoholic.org/. we thank you!

The meteor trace reflects radio waves like a mirror light. You can imagine it like this: the transmitter is a light source and the meteor track is a long mirror. So you notice a light spot on the mirror.

All observational data are based on this principle. The position of the light spot depends only on the angle of reflection. In our case, it depends on the position of the transmitter, receiver and from the track position.

The mechanism of the reflection depends on it, such as dense ionized gas traces. When the ion density is very high, the trace has plasma properties. Radio waves “they cannot enter” inside the track (the plasma glows at a higher frequency than the radio waves we use) and are reflected from “surface footprints”. We call such a trail dense (overdense trail). When plasma glows at a lower frequency than our observation frequency, radio waves penetrate the inside of the trace and scatter from lone electrons. We rarely call such a clue (underdense trail).

The reflection from the meteor trail is actually interference. The waves are scattering in all directions, but interference will cause, that the waves are most reflected in the direction of the law of reflection. Sometimes, however, the meteor trail deforms and can happen, that the signal is reflected from several points. This manifests itself as a phase difference between these signals. So we can determine, which part of the track contributes to the reflection, and which tends to dampen reflection. We call such parts of the trail Fresnel zones.

Thus, during a meteor flight, we register alternating signal amplification and attenuation. If we know the size of the Fresnel zones, the speed of the meteoroid flight can be found simply by the frequency of the oscillation. The size of the Fresnel zones depends on the angle of reflection and the frequency used.

Trace diffusion
Already during the formation of the trace, the ions begin to disperse into the surrounding atmosphere. The ion density distribution is always “Gaussian”.

Diffusion causes, that the dense part of the footprint increases, but its density gradually decreases. Then the size of the dense footprint decreases, when this trail disappears. This will also affect the received signal. First, the signal is amplified (the dense footprint increases), then weakens (the dense footprint decreases) and eventually disappears sharply (the thick trail disappeared). Only a thin trail remains.

With a sparse trace, the signal is reflected from isolated electrons. How the sparse trace disperses, coherence is lost and signal strength drops sharply. The decrease in signal strength is exponential. The exponential time constant depends on the diffusion rate, the diffusion rate is essentially a function of atmospheric density.

Influence (cut) wind
Strong wind in the upper atmosphere will distort and “tears” stop. You could also write, that it breaks the mirror.

The result is multiple reflections from different points on the track. Radio waves from different points of reflection are subject to interference. The wind is also to blame, that the point of reflection is not stationary, it manifests itself as a strong oscillation of the received signal. We won't notice this for a few seconds. So it's clear, that they can't do thin traces of this (they rarely last longer than a few tenths of a second). The usual frequency of this oscillation is 5 to 10 Hz. We sometimes call these oscillations “big leak – deep fading”.

Computer type recognition of tracks
I remind you again, that I have no experience with observing and processing radio meteors, so it's just a translation of a foreign website. If I gain the necessary experience, I like to brag …

If we use a computer to process observations (for example according to instructions developed at the University of Ghent), we can easily determine, types of footprints. This image shows a typical profile of a sparse meteor. A sharp increase is followed by an exponential decrease due to diffusion. (I took these pictures from www.imo.net)

The next picture is a typical dense meteor profile. Signal pulsations occur after a sharp rise in the signal (the cause is described above) and finally the signal drops sharply exponentially (the dense part of the trail disappears).

The next picture is a profile of a long-lasting dense meteor. Signal “looks like” similar to the previous case, but after a few seconds the wind will create multiple reflections, which in this case will create a large leak. Take good note of the scale in these two pictures.

I remind you, that not all meteor profiles are like that “nice” as in these pictures …

Conclusion
Based on observed “profiles” meteors should not be a problem to obtain the physical properties of meteors. But the meteor profile is also affected by other effects, which I did not deal with in the previous text. For example, ion density changes not only by diffusion but also by ion recombination. The rate of ion recombination depends on it, as the trail is illuminated by the Sun.. In addition, the characteristics of the atmosphere change rapidly and unpredictably. Therefore, we can seldom determine the exact angle of reflection. If we want to accurately analyze the properties of meteors, we should first accurately analyze these effects and meteor observations to correct these effects.

You can find the original article at http://meteory.persoholic.org/.