  We study the ionopause of Mars using a database of 6,893 ionopause detections
obtained over 11 years by the MARSIS (Mars Advanced Radar for Subsurface and
Ionosphere Sounding) experiment. The ionopause, in this work, is defined as a
steep density gradient that appears in MARSIS remote sounding ionograms as a
horizontal line at frequencies below 0.4 MHz. We find that the ionopause is
located on average at an altitude of $363 \pm 65$ km. We also find that the
ionopause altitude has a weak dependence on solar zenith angle and varies with
the solar extreme ultraviolet (EUV) flux on annual and solar cycle time scales.
Furthermore, our results show that very few ionopauses are observed when the
crustal field strength at 400 km is greater than 40 nT. The strong crustal
fields act as mini-magnetospheres that alter the solar wind interaction and
prevent the ionopause from forming.
