Very High Resolution Studies of Micrometeors Using the Arecibo 430 MHz Radar

Abstract We present measured and inferred properties of a possible new class of “sporadic” micrometeors discovered during 18 January 1995 observations made using the very sensitive 430 MHz radar system located at Arecibo Observatory in Puerto Rico. Over 200 of these objects were observed in a 2-hr period near sunrise. The average speed was about 55 km/sec in a range of 45–63 km/sec. Approximately two-thirds of the observed trajectories were apparently nearly parallel with the vertical beam and occurred on the 93–102 km height interval. The observed occurrence rate of these meteor returns combined with the size the Arecibo beam points to a meteor flux corresponding—in the “classical” view—to ∼15th magnitude micrometeors. This information along with observed deceleration rates and radar scattering cross-sections of order 10−8m2, leads us to conclude that the majority of the meteors observed appear to be of order 1 μg in mass. The depth of atmospheric penetration and inferred perihelia, the majority of which lie mostly within the orbits of Mercury and Venus, point to compositions of dense refractory material. Retrograde orbits that lie well out of the plane of the ecliptic combined with the modeled effects of radiation pressure induced orbit decay suggest that these particles—with no obvious parent body—originated in the outer reaches of the solar system and that they may even be primordial in origin. It is suggested that most if not all of these particles are associated with the North Apex “source” of sporadic meteors reported by Jones and Brown (1993,Mon. Not. R. Astron. Soc.265, 524–532). Additionally, several possible radar scattering mechanisms are discussed, none of which seem completely satisfactory.

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