Determining the initial radius of meteor trains: fragmentation

Abstract

The initial radius of meteor ionization has significant effects on measured height distributions, velocity distributions and flux measurements of underdense echoes determined from meteor radar observations. Multifrequency radar observations are used to examine the effects of initial train radius. A model has been constructed to explain the observed distribution, and has been tested on the 2001 Geminids. It is shown that fragmentation accounts for the most significant part of the attenuation due to finite train size. Knowledge of the initial radius of meteor trains and its variation with altitude is crucial for the determination of the fluxes and the orbital distribution of meteoroids because the initial train radius is one of the most important factors determining the attenuation of radar meteor echoes at a given wavelength. Radar observations in the VHF range are convenient for studying meteors and hence meteoroids since they can be made at any time of the day, in any kind of weather, and can record echoes from large numbers of meteors automatically. While a continuously running meteor patrol radar can produce accurate rates of meteor echoes, the fluxes can only be determined accurately if all observing biases are accounted for. While the limiting magnitude and collecting area for a given source are straightforward to calculate and contribute little error to the fluxes, it is difficult to estimate the correction that must be applied to the raw fluxes for the destructive interference resulting from the non-zero initial radius. When a meteoroid encounters the atmosphere, it produces a train of ionization several kilometres long. A typical meteor radar detects specular reflections from these trains, most of which are underdense because the train is radiatively thin and radiation is scattered from the entire cross-section of the train. Overdense echoes, which are caused by larger meteors, reflect radiation mainly from the surface of the cylinder of ionization and do not suffer from initial-radius effects. For infinitely thin trains there would be no height-dependent attenuation, but, immediately on formation, the train begins to expand and quickly attains an initial radius much larger than the meteoroid in the interval that the ablated ions are thermalized. Echoes from trains of radius comparable with the radar wavelength are significantly attenuated as a result of the lack of phase coherence from the signals reflected from the different parts of the train cross-section. Since the atmosphere is less dense at larger heights, the initial radius