  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. We show that the magnetic pressure usually dominates the
thermal pressure to hold off the solar wind in the ionopause at Mars, with only
13\% unmagnetized ionopauses observed over a 11-year period. We also find that
the ionopause altitude decreases as the normal component of the solar wind
dynamic pressure increases. Moreover, our results show that the ionopause
thickness at Mars is mainly determined by the ion gyromotion and equivalent to
about 5.7 ion gyroradii.
