Introduction

Current VHF/UHF radar micrometeor studies at Arecibo Observatory (AO), Puerto Rico, utilize the highly resolved (in time and height) meteor "head-echo"(as contrasted with classical meteor radars that detect the "trail-echo") to obtain meteor altitude, velocity and deceleration [1-3]. In addition, very precise beam pointing information provides good radiant information of large numbers of interplanetary dust particles (IDPs) [1] entering the earth atmosphere. The dynamical masses inferred using this approach appear to range from a small fraction of a nanogram to a few micrograms (< 1 - 100 /am diameter). These results combine to make AO uniquely situated and suited for the study and understanding of the IDP near 1 AU using ground-based observations.

Top-of-atmosphere velocity and mass estimates are obtained by integrating upwards the meteor mass and momentum equations. For this, we utilize the MSIS-E-90 model atmosphere (see http://www.wdc.rl.ac.uk/wdccl/ msis90.html). In addition, to test the effect of possible mass-loss, an empirical sputtering model with a sputtering yield that increases with energy [4,5] was adopted. However the orbital results discussed in the next section do not statistically change if our empirical model is neglected or if the in-

atmospheric meteor speeds are taken as the velocity at infinity. The Geocentric and heliocentric velocities are then determined and they include rigorous correction for diurnal aberration, zenith attraction and earth orbit ellipticity. The determination of the immediate (unevolved) meteoroid orbits follows from the determination of the meteor radiant and extra-atmospheric velocity [6].

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