Unusual features in high statistics radar meteor studies at EISCAT

Abstract

We describe results of an experiment conducted with the European Incoherent Scatter (EISCAT) radars during three 8-h runs on consecutive nights in 2008 December aiming to detect and study the high-altitude meteor population along with the meteors detected at classical ∼100-km altitudes. The experiment used coaxial ultra-high-frequency (UHF) and very high-frequency (VHF) radar beams pointed vertically to the zenith of Ramfjordmoen near Tromsø (Norway), and remote UHF receivers at Kiruna (Sweden) and Sodankylä (Finland) for tristatic observations of a very limited volume at an altitude of 170 km above the transmitter site. The EISCAT VHF radar detected during the 24-h period 22 698 echoes identified as meteors. The number of UHF echoes in the same period was 2138, most detected also at VHF. Among the VHF meteors, 11 were detected at altitudes higher than 150 km. Of these, the record highest meteor was at 246.9 km. No high-altitude UHF echoes were detected, none was tristatic, and no echoes with a Doppler velocity above ∼60 km s-1 were identified. Given the large number of echoes, which argues in favour of a highly significant characterization of the meteoroid population, we discuss the statistical properties of the detections and their possible physical nature. The average detection rate of VHF radar meteors was about 16 min-1. Comparing this high rate with that of the faintest optically detected meteors indicates that the radar detections originate from a meteoroid population that could be as optically faint as 13-14 mag. We did not observe a marked enhancement of the rates at the peak of the Geminid shower, confirming once again the proposal that most faint meteors, be these radar or optical, belong to the sporadic population and not to a specific shower. For a few meteors, our data show definite deceleration and possible fragmentation. A simple calculation indicates that one of the detected meteoroids was a submillimetre body that fragmented when the ram pressure reached about 0.5 pascal. This is much lower than the pressure that fragments brighter cometary meteors, which is at least two orders of magnitude higher. © 2009 RAS.