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On 10 September 2017, the Mars Atmosphere and Volatile EvolutioN mission observed a particularly strong X-class flare. This paper will focus on observations made by Neutral Gas and Ion Mass Spectrometer (NGIMS) and the flare response detected by extreme ultraviolet monitor. We focus the data to the region of the upper atmosphere from 160 to 300 km and to 10 orbits before and after the flare. The flare peaked near 16:12 UTC with the closest periapsis pass from 17:30 to 17:54 UTC (Lee et al., 2018, https://doi.org/10.1029/2018GL079162). NGIMS measured a significant enhancement in the neutral densities above 195 km for the flare. This enhancement stands out for the major species measured by NGIMS (Ar, CO2, CO, O, and N-2). The correlation of the flare and the enhancement in density and temperature in the upper atmosphere indicates that solar flare heating is most likely the main driver and has important implications for the effects of space weather events on terrestrial atmospheres.

Plain Language Summary 10 September 2017 saw an extraordinary solar flare event that affected both Mars and Earth, providing an unique observational opportunity. It has been observed in the past that solar flares can cause tremendous rapid heating of the upper atmosphere of the Earth, but the extent of this heating on the neutral atmosphere is yet to be well observed at Mars. Early MAVEN observations caught smaller flare effects and observed rapid heating to a lightly lesser extent. The 10 September 2017 flare is not only significantly more intense than previous observations, but the timing of neutral observations was optimal to observe just how much and how quickly the upper atmosphere responded to the flare. This can lead to refinement of atmospheric escape and atmospheric evolution.