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Martian Thermospheric Response to an X8.2 Solar Flare on 10 September 2017 as Seen by MAVEN/IUVSWe report the response of the Martian upper atmosphere to a strong X-class flare on 10 September 2017 as observed by the Imaging Ultraviolet Spectrograph (IUVS) instrument aboard the Mars Atmosphere Volatile EvolutioN (MAVEN) mission. The solar flare peaked at 16:24 hr UT, and IUVS dayglow observations were taken about an hour after the flare peak. Retrieved temperatures from IUVS dayglow observations show a significant increase during the flare orbit, with a mean value of similar to 270 K and a maximum value of similar to 310 K. The retrieved temperatures during the flare orbit also show a strong latitudinal gradient, indicating that the flare-induced heating is limited between low and middle latitudes. During this event IUVS observed an similar to 70% increase in the observed brightness of CO2+ ultraviolet doublet and CO Cameron band emission at 90 km, where high-energy photons (< 10 nm) deposit most of their energy. Plain Language Summary We report here first observations of thermospheric response to an X8.2 class flare on Mars measured by Imaging Ultraviolet Spectrograph on board the Mars Atmosphere Volatile EvolutioN spacecraft. We found that thermospheric temperatures increased by (similar to)70 K at the time of the flare, although IUVS observations took place roughly 1 hr after the flare peak. The thermospheric temperatures retrieved by IUVS show that the flare-induced heating was limited to the low and middle latitudes only. This is the first reported observation of latitudinal extent of atmospheric heating caused by a solar X-class flare on Mars.