Disentangling Seasonal and Solar EUV Dependences of Martian Thermospheric Densities

Abstract:

We present an analysis of the long-term variability of Martian thermospheric densities within 150-200 km altitude using observations of the NASA Mars Atmosphere and Volatile EvolutioN (MAVEN) mission over half a solar cycle. Neutral CO₂, N₂, O, and Ar densities are obtained from in-situ measurements by the MAVEN Neutral Gas and Ion Mass Spectrometer (NGIMS) instrument during periapsis passages and averaged over 27-day intervals to remove longitudinal variations and solar rotational effects. These neutral measurements, together with concurrent observations of the overhead solar EUV intensity from the MAVEN Extreme Ultraviolet Monitor (EUVM), allow us to disentangle the seasonal and solar EUV dependences of thermospheric densities. The long-term variability is well captured by empirical functions from a least-squares fit, from which the contributions and competitions of seasonal and solar EUV effects are revealed in their control of the thermospheric density distributions. It is found that the seasonal change of the Mars thermosphere is strong and correlates well with the Sun-Mars distance change. In general, we see minimum densities near aphelion and maximum densities near perihelion. The solar activity term provides an additional important modulation. Altogether, thermospheric densities are subject to considerable long-term variation by up to an order of magnitude. Within this long-term variability, the potential influence from dust storm activity is also explored and quantified.