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 similar to 1-2 x 10(4) 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. Plain Language Summary The solar flare that occurred on 10 September 2017, and the subsequent coronal mass ejection had major impacts on the upper atmosphere of Mars. In this work, we study how the Martian ionosphere, which is the ionized part of the upper atmosphere, changed in response to this prominent solar event. Analysis of data from two spacecraft, Mars Express and MAVEN, reveals unusually strong magnetic fields everywhere in the ionosphere and an exceptionally high level of ionization during the night time. These results can be compared and combined with other observations and models and help the ongoing community wide effort to understand the effects of the solar event on planetary environments.