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MARSIS Observations of the Martian Nightside Ionosphere During the September 2017 Solar Event

We present topside ionospheric sounding on the nightside of Mars during the September 2017 solar event by Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) on board Mars Express along with complementary dayside observations from Mars Atmosphere and Volatile EvolutioN (MAVEN). The MARSIS and MAVEN observations during the event suggest that (i) the nightside bottomside ionosphere was significantly enhanced by solar energetic particles, (ii) the nightside peak electron density was increased to unusually high values of ∼1–2 ×104 cm−3 around 120 km altitudes owing to enhanced electron impact ionization, and (iii) the ionospheric magnetic field was globally amplified by the high dynamic pressure during the interplanetary coronal mass ejection passage. These multipoint measurements help elucidate the global response of the Martian upper ionosphere to the solar event.