Overview of MEX observations of the Martian ionospheric response to the September 2014 space weather event. Time series data of (a) 10-min averaged counts of Mars Express IMA in the highest energy channel representing the background level, which can be used as a proxy for SEP protons (Futaana et al. 2008), (b) 5–5.5 MHz surface echo power measured by MARSIS, the reduction of which indicates radio absorption in the lower ionosphere of Mars, derived in the same manner as (Harada et al. 2023), (c) range spread of 1.4–1.6 MHz ionospheric echoes derived from periapsis MARSIS observations, showing the level of ionospheric irregularities in the topside ionosphere of Mars (Harada et al. 2018a), and (d) solar zenith angle of Mars Express. Panel e: geographic map of periapsis MARSIS observations, with colors indicating the range spread of 1.4–1.6 MHz ionospheric echoes. Radargrams demonstrate the range spread of 1.4–1.6 MHz ionospheric echoes for orbits (f) 13549 and (g) 13595.