The Martian Magnetosphere - A Laboratory For Bi-Ion Plasma Investigations

Abstract

The Martian space gives us an example of the direct interaction of the solar wind dominated by protons and the planetary plasma of heavy ions. Due to the lack of the intrinsic magnetic field the solar wind exposes the extended exosphere/atmosphere of Mars far from the bow shock. Photoionization, electron impact and charge exchange lead to the creation of newly born ions which may affect the incoming solar wind. At large distances the most abundant neutral components are molecular and atomic hydrogen, which could reach very high altitudes as the consequence of a weak gravitational attraction of Mars. It was found that pickup protons modify significantly the structure of the ion foreshock and, probably, the bow shock itself due to their effective reflection at the bow shock (Dubinin et al., 1994; Dubinin et al., 1995). Fast oxygen atoms originated in the process of the dissociative recombination of molecular oxygen, which dominates at the ionosphere heights additionally contribute to the neutral exosphere. Closer to the planet, the shocked solar wind interacts with a dense oxygen atmosphere, which produces an effective obstacle to the solar wind. In addition to the features related with the extended atmosphere, the importance of the ion gyroradius effects is also unique at Mars. The pick-up gyroradius of O + ions exceeds the characteristic size of the system and the classical MHD description is violated. In such Large Larmour Radius (LLR) configurations a relative streaming of different ion species is admissible and leads to a strong coupling between the solar wind protons and heavy planetary ions. In this paper we focus on effects of heavy ion population on dynamics of the solar wind. In section 2, a description of the MHD approach for two ion species is given. In section 3, we discuss mechanism of coupling between both species moving with different speeds. In section 4, the problem of critical points and shocks in bi-ion flows is considered. In section 5, we briefly discuss phenomenon of the’ obstacle boundary’ near Mars. In section 6 we derive the dispersion relations for low-frequency waves in bi-ion plasma composed of the solar wind as a core plasma and a minority of unmagnetized heavy ions which are in a relative streaming. In section 7, dispersion analysis of whistler waves is made. Application of these results to the Martian space is given. Observations and results of modeling are presented to illustrate our findings.