The Dayside Ionopause of Mars: Solar Wind Interaction, Pressure Balance, and Comparisons with Venus

TL;DR

This study investigates the solar wind interaction and pressure balance at Mars's ionopause, revealing magnetic dominance in pressure support and comparing ionopause characteristics with Venus.

Contribution

It provides the first comprehensive analysis of Martian ionopause pressure balance using in situ and remote data, highlighting magnetic pressure dominance over thermal pressure.

Findings

Magnetic pressure usually dominates at Mars ionopause.

Ionopause altitude decreases with increased solar wind pressure.

Ionopause thickness is about 5 ion gyroradii, influenced by ion gyromotion.

Abstract

Due to the lower ionospheric thermal pressure and existence of the crustal magnetism at Mars, the Martian ionopause is expected to behave differently from the ionopause at Venus. We study the solar wind interaction and pressure balance at the ionopause of Mars using both in situ and remote sounding measurements from the MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) instrument on the Mars Express orbiter. We show that the magnetic pressure usually dominates the thermal pressure to hold off the solar wind at the ionopause at Mars, with only 13% of the cases where the ionospheric thermal pressure plays a more important role in pressure balance. This percentage at Venus, however, is up to 65%. We also find that the ionopause altitude at Mars decreases as the normal component of the solar wind dynamic pressure increases, similar to the altitude variation of the ionopauses at Venus. Moreover, our results show that the ionopause thickness at Mars and Venus is mainly determined by the ion gyromotion and is equivalent to about 5 ion gyroradii.