Mars Thermospheric Variability Revealed by MAVEN EUVM Solar Occultations: Structure at Aphelion and Perihelion and Response to EUV Forcing

The Mars thermosphereholds clues to the evolution of the Martian climate, and has practical implications for spacecraft visiting Mars, which often use it for aerobrakingupon arrival, or for landers, which must pass through it. Nevertheless, it has been sparsely characterized, even when past accelerometer measurements and remote observations are taken into account. The Mars Atmosphere and Volatile EvolutioN (MAVEN) orbiter, which includes a number of instruments designed to characterize the thermosphere, has greatly expanded the available thermosphericobservations. This paper presents new and unanticipated measurements of density and temperature profiles (120-200 km) derived from solar occultationsusing the MAVEN Extreme Ultraviolet(EUV) Monitor. These new measurements complement and expand MAVEN's intended thermosphericmeasurement capacity. In particular, because the local-timeis inherently fixed to the terminator, solar occultationsare ideally suited for characterizing long-term and latitudinal variability. Occultationmeasurements are made during approximately half of all orbits, resulting in thousands of new thermosphericprofiles. The density retrieval method is presented in detail, including an uncertainty analysis. Altitude-latitude maps of thermosphericdensity and temperature at perihelionand aphelionare presented, revealing structures that have not been previously observed. Tracers of atmospheric dynamics are also observed, including a high altitude polar warming feature at intermediate latitudes, and an apparent thermostaticresponse to solar EUV heating during a solar rotation, which shows heating at high altitudes that is accompanied by cooling at lower altitudes.