Meteor Astronomy with Forward Scatter

In this section, the astronomical aspect of the problem is introduced: what do meteor astronomers want to know about meteors, how should these parameters be measured with radio meteor installations and which projects are radio set-ups well-suited for?

Meteor parameters of interest

When observing meteors, one can focus on the study of individual meteors, the study of meteor streams or that of the sporadic background.

For the study of individual meteors the parameters to be determined are the orbit of the particle and its mass. Orbits can be calculated when the radiant, velocity, and deceleration of meteors are determined. The mass can be estimated by measuring the ionization profile of the trail.

For streams, the particle number density, the mass distribution, the radiant and the velocity of the stream meteors are of interest. The structure of the stream can be determined by analyzing the evolution of these parameters while the Earth crosses the stream. The particle density is related to the meteor rates, and the mass distribution can be derived by statistically analyzing the proportion of “big” meteors to “small” ones.

The study of the sporadic background can be considered as the study of a big number of streams with radiants distributed all over the sky. Besides the particle density and mass distribution, the radiant distribution and the velocity distribution should be analyzed.

Measurement of the meteor parameters by radio

In the case of radio meteor observations, the determination of the relevant physical parameters, which were enumerated in the previous subsection, often has to be done indirectly. Generally, a series of apparently unrelated observational parameters have to be determined in order to be able to obtain the wanted physical quantities. Due to the very indirect measuring techniques used, the quality of the theory used essentially determines the quality of the obtained results.

In Section “Practical Information”, the way in which these parameters can be determined in practice is discussed for different kinds of set-ups.

It is of course always supposed that the observations are “clean,” i.e., that only one transmitter is used, and that interferences and other disturbing signal sources are absent or can be identified and compensated for.

Some specific projects for radio meteor set-ups

The advantage of radio observations with respect to other meteor observation techniques is the mass range covered on one hand, and the possibility to observe at almost any moment on the other. Projects for radio set-ups are generally based on these advantages.

One parameter to determine when studying small meteoroids is the mass index, to compare it with the mass index of the heavier particles. Other points of interest for small meteoroids are for instance the ablation height and the ionization profile, as for smaller particles the classical ablation theory – that predicts certain height distributions and ionization profiles – does not stand any more, and the more appropriate “dustball theory” still needs some parameterization [5]. The examined mass range can be modulated by changing the gain of the antenna systems. High gains favor the study of the smaller particles (but introduce a strong biasing of the observed meteors, based on their spatial position and orientation), while low gains can be used to concentrate the observations
on the larger meteoroids.

The continuity of the observations makes radio meteor systems ideal for the discovery or confirmation of meteor outburst. The clear observation of the α-Monocerotids by several amateur forward scatter systems on November 22, 1995, is an excellent example of this (see e.g. the graphs in [21]). Of course, the stream producing an outburst should be observable by the set-up at the moment of the outburst. Therefore, set-ups with a good sky coverage are highly preferred for this kind of “burst monitoring.”

The daylight meteor streams have not been observed well for years now, so there is need for a new monitoring of these streams to determine their activity level and moment of maximal activity. For this, it is interesting to be able to observe the stream well for several hours on end, to be able to detect variations in the stream parameters. Consequently, a good sky coverage is useful here too.