On the role of charge exchange in the formation of the Martian magnetic pileup boundary

Abstract

In the magnetic pileup boundary (MPB) of Mars a steep increase in the magnitude of the magnetic field is observed in the data from the magnetometer on board Mars Global Surveyor, and the growth of the field is more precipitous than common solar wind‐ionosphere models predict. Analysis of both Venus and Mars magnetic field data has strongly implied that plasma pressure is exchanged for magnetic pressure in the magnetic pileup field. To explain this, effects of the Martian exosphere must be included. Then charge exchange between the shock‐compressed solar wind protons, which dominate the thermal pressure, and neutral exospheric atoms (mainly H and O) should play a crucial role in the formation of the MPB, as is also inherently required by the conservation of momentum flux. To test this idea, a model of the solar wind interaction with the Martian upper atmosphere, with charge exchange included, is constructed to reproduce the observed magnetic field of the MPB of Mars. The simulation results show that charge exchange is able to account for the sudden buildup of the magnetic field in the Martian MPB, with the implication that the same mechanism produces the magnetic field increase at Venus and comets.