The structure and variability of Mars dayside thermosphere from MAVEN NGIMS and IUVS measurements: Seasonal and solar activity trends in scale heights and temperatures

Mars dayside thermospheric temperature and scale height trends were examined using measurements from the Neutral Gas Ion Mass Spectrometer (NGIMS) and the Imaging Ultraviolet Spectrograph (IUVS) on the Mars Atmosphere Volatile EvolutioN (MAVEN) spacecraft. Average scale heights (over 150–180 km for solar zenith angles ≤75°) from several different sampling periods were obtained from each instrument. NGIMS and IUVS scale height trends were found to be in good agreement, with both showing scale heights decreasing after perihelion and reaching a low value near aphelion (13.6 to 9.4 km). Between these two seasonal extremes, the temperature decreased by ∼70 K (from 240 to 170 K). These trends were also analyzed with respect to the changing solar flux reaching the planet, using the Lyman alpha irradiance measured by the Extreme Ultraviolet Monitor (EUVM) on MAVEN. Scale heights responded strongly to the changing solar flux. During this part of the MAVEN mission (October 2014 to May 2016), it was concluded that over longer timescales (at least several months), dayside thermospheric temperatures are chiefly driven by changing solar forcing, although it is the effects of changing heliocentric distance rather than changing solar activity which seem to have the greatest impact. Furthermore, effects of solar forcing were not observed on shorter timescales (less than a month), suggesting local wave effects may dominate solar forcing on these timescales. Finally, temperatures from two NGIMS sampling periods were compared to temperatures from the Mars Global Ionosphere-Thermosphere Model (M-GITM) and found to be in good agreement.