The Martian magnetosheath: how Venus-like?

The planets Mars and Venus, because of their weak global magnetic fields, have small-scale magnetosheaths amenable to detailed analysis and model comparisons. In this paper we examine some of the similarities and contrasts between the Venus and Mars cases based on Pioneer Venus Orbiter (PVO) magnetometer observations from the PVO prime mission, and Mars Global Surveyor magnetometer observations obtained during the MGS Science Phasing Orbits (March-September, 1998). The combination of a mass-loaded magnetohydrodynamic magnetosheath model and a data-based model of the Martian crustal fields is used to illustrate the differences produced by the presence of the Martian crustal fields. While Venus at solar maximum exhibits a nearly classical magnetosheath formed by the solar wind interaction with a practically impenetrable blunt body, Mars in late 1998 represents a complicated obstacle whose own magnetic fields compromise this simplicity within at least several hundred km of the nominal obstacle boundary inferred from the bow shock position. In particular, the results suggest the presence of a thick inner magnetosheath boundary layer when the strong southern hemisphere crustal fields are located on the sunward hemisphere. (C) 2002 Elsevier Science Ltd. All rights reserved.