Upstream ULF fluctuations near Mars

Abstract

Magnetic field data from the Phobos 2 spacecraft near Mars (March 1989) are investigated for ULF fluctuations in the upstream region of the solar wind plasma onstreaming onto the planet. In general, ULF fluctuations of the magnetic field are expected when the spacecraft is located in the ion foreshock, which is defined through the actual magnetic field direction. A statistical study of the occurrence of such ULF fluctuations for all circular orbits around Mars is presented. The upstream boundary of the foreshock is found at 9BN ≈61° for moderate values of the angle between the solar wind velocity vector vsw and the magnetic field vector B; for more perpendicular magnetic field configurations, the boundary lies at approximately the same value of 9BN ≈ 60°. At Mars these boundaries occur at higher 9BN values than at Earth. Higher turbulence is found deeper in the foreshock. Therefore the situation is qualitatively in accordance with the situation at Earth or Venus. One difference occurs in the region upstream of the foreshock, where Mars seems to have more wave activity. The reason for this difference is unclear but may be related to the role of planetary ions in the foreshock or to the contribution of ion kinetic effects to the solar wind interaction with Mars. Copyright 1998 by the American Geophysical Union.