A radio science perspective on long-duration meteor trails

摘要

Nonclassical radar meteor echoes or range spread trail echoes (RSTEs), lasting from tens of seconds to over 15 min, have been a subject of considerable interest and speculation in the community ever since they were first observed in the 1940s. Using data collected from the Jicamarca Radio Observatory 50 MHz VHF radar in June 2006, we find that many properties of RSTEs can be explained largely from a radio science perspective. On a statistical basis, RSTEs of duration >15 s are observed exclusively from scatterers in the k B (k = radar wave number; B = geomagnetic field) region apparently as the result of trail evolution parallel to B, forming approximately Fresnel-zone sized scattering regions thus narrowing the scattering pattern. This result implies that the initial irregularity structures in the meteor trail exhibit a wide scattering pattern that can be seen from anywhere in the radar beam given sufficient radar sensitivity but once the trail significantly elongates along B, it can be detected only from the k B region of the radar. This conclusion has far-reaching implications to current interpretations of instability development in RSTEs as the location of the meteoroid trajectory relative to the narrow k B region strongly determines observed RSTE properties such as onset time relative to the head echo and trail lifetime as functions of altitude. That is, a RSTE event viewed by two closely spaced identical radars would have different properties. Meteoroid size, energy, and fragmentation as well as radar properties such as frequency, beam pattern, and absolute sensitivity play obvious roles as well.