Nonlinear MHD waves and discontinuities in the Martian magnetosheath. Observations and 2D bi-ion MHD simulations

Abstract

The characteristic scale of the Martian magnetosheath is less than the pick-up gyroradius of oxygen ions. This leads to admissible differential motion of protons and heavies and a strong coupling between both ion fluids. 2D biion MHD simulations reveal many new interesting features in such Large Larmour Radius systems. The formation of an ion-composition boundary, which separates both plasmas, and structuring of the transition from proton dominated plasma of the solar wind origin to massive planetary plasma are the main features of the interaction. A comprehensive multi-instrument study of Martian plasma environment and the comparison with theoretical modelling initiated in the framework of the Visiting Science Programme of the International Space Science Institute (ISSI) in Bern (Switzerland) gives confirmation that Mars interacts with the solar wind like a comet which has a outgassing rate near to that of Grigg-Skjellerup. The results may also be relevant for small bodies which are surrounded by a neutral gas atmosphere (icy moons, asteroids, Mercury).

Modeling and Simulating Flowing Plasmas and Related Phenomena

“…The multi-fluid MHD equations include the continuity, momentum and energy equations for each species (note this does not include the model of Sauer et al 1990Sauer et al , 1994Sauer et al , 1998. The number of equations increases significantly for the multi-fluid MHD model.…”

Hybrid code simulations of the solar wind interaction with Pluto

“…In the solar wind reference frame, the unmagnetized heavy ions appear as a beam distribution streaming through the solar wind. It has been shown extensively in the literature that such a configuration leads to the excitation of low‐frequency electromagnetic waves (LFEW) at roughly the proton gyrofrequency [ Sauer et al , 1996, 1998a, 1998b, 1999a, 1999b; Dubinin and Sauer , 1999; Baumgärtel et al , 1998].…”

“…The role of bi‐ion flows near Mars also lead to considerable discussion of waves and discontinuities in the Martian magnetosheath [ Sauer et al , 1998a, 1998b; Dubinin et al , 1996]. In particular, the contamination of the solar wind flow by secondary heavy ions can modify the bow shock ahead of the obstacle by causing a second discontinuity downstream of the bow shock, called the heavy ion discontinuity [ Sauer et al , 1996].…”

Multiple shocks near Mars

“…A motion of both ion populations begins gyrating and the exchange of momentum between ion flows occurs in a periodical manner. The process of alternating acceleration/deceleration of heavies/ protons becomes more clear if we write the momentum equations for protons and heavies in bi-ion plasma with differential streaming (Sauer et al, 1998): Indeed, the Lorentz force, related with a differential streaming of ion fluids (≈Δv × B), has the opposite action on protons and heavies. A "larmoring" of ion populations occurs because either ion fluid in their reference frame senses the motional electric field -(1/c)Δv × B which is supplied by differential ion streaming.…”

“…Heavy ion shocks take their origin from the slow modes which are carried by an ion beam and resonantly interact with proton waves. 2-D bi-ion MHD simulations (Sauer et al, 1998) allowed to trace these waves for the case of the solar wind interaction with Mars. "Proton shocks" give rise to a bow shock, and "heavy ion shocks" form additional MHD-discontinuities in the magnetosheath of Mars (Sauer et al, 1996b).…”