First results of the micrometeoroid experiment s 215 on the HEOS 2 satellite

The experiment measures the mass, speed and approximate influx direction of the micrometeoroids. They are detected by the plasma produced during their impact on the sensor. In the first year after launch 173 particles were registrated. The temporal distribution of the particles indicates three different categories. Category 1 consists of 3 large groups of particles ("swarms") each of which encountered the detector within some tens of minutes. Category 2 includes 8 small groups of particles each of which was detected within hours. Category 3 is made up by the remaining particles which are randomly distributed. The swarms are of very recent origin, possibly supplied by (1) meteoroids grazing the Earth's atmosphere, (2) ejecta from the Moon, and (3) collisions of dust particles. The groups may be dissipated swarms or particles from the outer zone of swarms. The randomly distributed particles consist of sporadic interplanetary particles and completely dissipated groups. The comparison of both the "particle rates" (number of detected particles per day) and the cumulative particle flux curves for the Earth's apex, antiapex, ecliptic north and south directions shows that the rates vary only within a factor of 2, whereas the particle flux is extremely anisotropic. The flux for the apex direction at 10 ^-12 g is 7 × 10 ^-5 m ^-2 s ^-1 which is 1 to 2 orders of magnitude higher than the flux values for the other directions.