Doppler studies of near-antapex UHF radar micrometeors

A "radar micrometeor" is the radar-scattering signature from the free elect rons in the plasma generated by entry of a dust-sized meteoroid into the at mosphere. We report the first direct Doppler measurements, made using the A recibo Observatory 430-MHz radar, of the so-called meteor head echo, Our ob servations demonstrate that this region is moving with the speed of the met eoroid as determined from the meteor head-echo altitude-time trajectory and that this radar return is distinct spatially and in velocity from the much more commonly observed trail echo. We also report the first observations o f near-antapex micrometeors which are characterized by the very slow atmosp heric speeds expected from low-ecliptic-inclination objects entering the at mosphere from behind Earth's orbital path. Of the 32 meteors observed durin g four early evening hours of observations on 10 January 1997, velocities w ere determined for 18 of the meteors of which 7 were at or just below Earth escape velocity (11.2 km/s), We give heliocentric orbits for the 11 meteor events with speeds greater than the escape velocity and present a detailed analysis of these orbital parameters and their possible origins. One parti cle was determined to be interstellar: a preliminary analysis indicates tha t the ecliptic coordinates of the radiant relative to the local standard of rest (LSR) (with the solar motion relative to the nearby stars removed) ar e lambda = 43.02 degrees, beta = -43.28 degrees, V = -25.11 km/s or, in sys tem II galactic coordinates l(II) = 219.8 degrees, b(II) = -52.4 degrees, V = -25.1 km/s. (C) 2000 Academic Press.