I was very interested in the following article and I asked its author - Ladislav Bálint - for permission to publish it on CQ.sk. Ladislavova stránka „Meteory“ je výborná. Ďakujeme!
A meteor trail reflects radio waves like a mirror reflects light. You can think of it like this: the transmitter is the light source and the meteor trail is a long mirror. So you will notice a point of light on the mirror.
All data derived from observations are based on this principle. The position of the light point depends only on the angle of reflection. In our case, it depends on the position of the transmitter, the receiver and the position of the track.
The reflection mechanism depends on how dense the tracer's ionized gas is. When the ion density is very high, the trace has plasma properties. The radio waves "cannot enter" the interior of the trace (the plasma glows at a higher frequency than the radio waves we used) and are reflected from the "surface of the trace". We call such a trail an overdense trail. When the plasma glows at a lower frequency than our observation frequency, radio waves enter the interior of the track and scatter from the lone electrons. We call such a trail an underdense trail.
The reflection from the meteor trail is actually interference. Although the waves scatter in all directions, the interference causes the waves to reflect the most in the direction according to the law of reflection. However, sometimes the meteor trail is distorted and it can happen that the signal is reflected from several points. This will show up as a phase difference between these signals. So we can determine which part of the track contributes to the reflection, and which part tends to dampen the reflection. We call such parts of the track Fresnel zones.
That is, during the passage of the meteor, we register an alternating strengthening and weakening of the signal. If we know the size of the Fresnel zones, the speed of the meteoroid can be determined simply by the frequency of oscillation. The size of the Fresnel zones depends on the angle of reflection and the frequency used.
You will read in the article
Trace diffusion
Already during the formation of the track, the ions begin to disperse into the surrounding atmosphere. The ion density distribution is always "Gaussian".
Diffusion causes the dense part of the trace to increase in size, but its density gradually decreases. Then the size of the dense trail decreases until the trail disappears. This will also affect the received signal. First the signal gets stronger (dense trace gets bigger), then it gets weaker (dense trace gets smaller) and finally it suddenly disappears (dense trace disappeared). Only a thin trace remains.
In a sparse trace, the signal is reflected from lone electrons. As the sparse trace dissipates, coherence is lost and signal strength drops sharply. The decrease in signal intensity is exponential. The exponential time constant depends on the rate of diffusion, the rate of diffusion being essentially a function of atmospheric density.
The effect (shear) of the wind
Strong winds in the upper atmosphere will distort and "tear" the trail. It could also be written that he 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 responsible for the fact that the reflection point is not stationary, it manifests itself as a strong oscillation of the received signal. We notice this only after a few seconds. So it is clear that sparse traces (they rarely last longer than a few tenths of a second) cannot do this. The usual frequency of this oscillation is 5 to 10 Hz. We sometimes call these oscillations "deep fading".
Recognizing types of footprints using a computer
Here I remind you again that I have no experience with observing and processing radio meteors, so this is just a translation of foreign websites. If I get the necessary experience, I'll be happy to show off...
If we use a computer to process the observations (for example, according to the instructions developed at the University of Ghent), we can easily determine the types of tracks. This image shows the typical profile of a rare meteor. A sharp increase is followed by an exponential decrease caused by diffusion. (I took these images from www.imo.net)
The next image shows a typical profile of a dense meteor. After a sharp rise in the signal, signal pulsations occur (the cause is described above) and finally the signal drops sharply exponentially (the thick part of the trace disappears).
The next image shows the profile of a long-lived dense meteor. The signal "looks" similar to the previous case, but after a few seconds the wind creates multiple reflections, which in this case create a large leak. Note the scale in these two images well.
I remind you that not all meteor profiles are as "pretty" as in these pictures...
Conclusion
Based on the observed "profiles" of the meteors, it should not be a problem to obtain the physical properties of the meteors. But the profile of the meteor is also affected by other effects, which I did not deal with in the previous text. For example, the ion density changes not only by diffusion but also by ion recombination. The rate of ion recombination depends on how the track is illuminated by the Sun. Moreover, the properties of the atmosphere change rapidly and unpredictably. Therefore, we rarely know exactly the angle of reflection. If we want to accurately analyze the properties of meteors, we should first accurately analyze these effects and correct meteor observations for these effects.