On the geocentric micrometeor velocity distribution

We report micrometeor velocity distributions measured with unprecedented velocity and radiant resolution using the dual-beam 430 MHz Arecibo (AO) radar in Puerto Rico. The AO radar detects over 10,000 events daily inside its 300 in radar beam that are produced mainly by particles in the size range 0.5-100 microns. During the observations reported here, the line feed antenna is pointed vertically while the Gregorian feed is pointed at an angle of 15 degrees from zenith. The off-vertical radar beam is initially placed pointing north and every 30 min is rotated 180 degrees, allowing observation of three different regions of the Earth's ionosphere. Results from the observations performed on 21 January and 25 June 2002 are presented and discussed. We observe that the meteoroid population detected by AO is smaller in particle size and faster in velocity and thus entirely different from the one observed by conventional lower-frequency meteor radars that use a different scattering mechanism. We observe qualitative difference in the shape of the meteor velocity distribution for the different pointing directions. Preliminary analysis of these distributions indicates that we detect at least four particle populations characterized by their geocentric velocities: A slow one with a "classical" value (∼15 km/sec), an intermediate velocity population (∼30 km/sec) the presence of which depends on ecliptic latitude and longitude, and two fast and dominant (at sunrise) populations (∼45 km/sec and ∼50 km/sec). Finally, we explore the possibility of observational biases in our technique and find no evidence for large effects.