Pergamon www.elsevier.nlllocateiasr zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Adv. Space Res. Vol. 25, No. 2, pp. 299402, 2000 0 IY9Y COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0273 1177100 $20.00 + 0.00 Pll: SO2731 177(YY)OO952-7 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONML SELECTION EFFECTS ON INTERSTELLAR DUST IN HELIOSPHERE H. Kimura and I. Mann Max-Han&-Institut fiir Aeronomie, Max-Plan&-.%$/e 2, D-37191 Katlenburg-Lindaz, Gemany ABSTRACT Detailed studies of the selection effects at work on interstelfar dust grains during their entry into the heliosphere are important deducing the conditions of grains in the very local interstellar medium. In p~ticular we study the influence of Lorentz and radiation pressure forces on the interstellar dust flux. Lorentz forces acting on grains se- lectively deplete smaller sized grains from their original flow near the heliopause and in the heliosphere, irrespective of dust composition. If interstell& grains in the very local interstell~ medium are composite fluffy a~lomerates as proposed for grains in the diffuse interstellar medium, then the radiation pressure force is su~~iently small that the size distribution and material composition can be retrieved from in situ measure~ients in the solar system. Although grains that consist only of silicate are scarcely influenced by radiation pressure, the flux of bare graphite grains is expected to decrease by radiation pressure in contrast to the constant Aux derived from in situ me~urements of grains in the heliosphere. It is unlikely that interstellar grains are destroyed by field emission or sputtering, even though the grains pass through shocks to enter the heliosphere, and by mutual collisions or collisions with interplanetary dust particles. 01999 COSPAR. zyxwvutsrqponmlkjihgfedcbaZYXWV Published by Elsevier Science Ltd. INTRODUCTION The motion of the solar system relative to the very local interstellar medium (VLISM) gives us the opportunity to study the dynamical, physical, and chemical properties of interstellar dust within the heliosphere. In situ measure- ments of cosmic dust performed by the Ulysses, Galileo, and Hiten spacecraft have proven that interstellar dust grains penetrate the heliosphere (Griin et aI., 1994, 1997; Svedhem et al.? 1996). A detailed analysis of the Ulysses meaSure- ments yields further information on the size distribution and mass density of interstellar dust between l-5 AU from the Sun (Griin et aE., 1994; Landgraf, 1998). Sample return missions, such as zyxwvutsrqponmlkjihgfedcbaZYXWVU STARDUST, will reveal the physical and chemical properties of interstellar dust grains which enter the solar system. While data for these particles will be used to retrieve the original conditions of interstellar dust in the VLISM, the size and spatial distributions of the interstellar dust flux are probably modified by Lorentz and radiation pressure forces in the lleliosphere (Mann, 1996). Therefore the retrieval of the original information requires a knowledge of the effects which selectively prevent certain types of interstellar dust from entering the heliosphere. We study the selection effects operating on interstellar dust flows into the heliosphere, mainly based on the estimates of the Lorentz and the radiation pressure forces. MODELS FOR INTERSTELLAR DUST While many researchers have derived models of the dust in the diffuse interstellar medium (ISM) based on observations of interstellar extinction and polarization, in situ measurements in the heliosphere increase directly our knowledge of dust in the VLISM. On the basis of the data analysis of the U@ses in situ measurements by Wehry and Mann (1999), the mass density of interstellar dust in the heliosphere has recently been shown to be consistent with the mean density in the diffuse ISM (Kimura zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA et al., 1999). We may expect that the dynamical, physical, and chemical properties of interstellar dust in the VLISM are similar to those in the diffuse ISM. It is therefore worth applying currently available zyxwvutsrq 299