Variability of Mars Thermospheric Neutral Structure from MAVEN Deep Dip Observations: NGIMS Comparisons with Global Models

The Mars upper atmosphere (~100-500 km), encompassing the thermosphere, ionosphere, and lower portion of the exosphere, constitutes the reservoir that regulates present day volatile escape processes from the planet (Bougher et al. 2014). One of the key strategies of the MAVEN primary mission, central to the exploration of this reservoir region, is to execute several Deep Dip campaigns. During each week-long campaign, periapsis is lowered from a nominal altitude of ~150-170 km down to ~120-135 km. This strategy allows direct in-situ sampling of the entire reservoir region, from the exosphere downward to near the homopause (Jakosky et al. 2015). This presentation focuses on the neutral thermospheric structure sampled during the first three Deep Dip campaigns #1 , #2 and #3, corresponding to dusk (mid-Northern latitudes), subsolar, and dawn (mid-Southern latitudes), locations respectively. The primary MAVEN instrument for sampling neutral thermospheric structure is the NGIMS (Neutral Gas and Ion Mass Spectrometer) instrument. It measures the neutral composition of the major gas species (e.g. He, N, O, CO, N2, O2, NO, Ar and CO2) and their major isotopes, with a vertical resolution of ~5 km for targeted species and a target accuracy of <25% for most of these species (Mahaffy et al. 2014). Thermospheric temperatures are derived from the neutral scale heights. Corresponding Mars Global Ionosphere Thermosphere Model (MGITM) outputs are subsequently compared to NGIMS neutral composition and temperatures along the inbound legs of orbit tracks below 300 km. This MGITM model is primarily driven by solar EUV-UV forcing at thermospheric altitudes (Bougher et al. 2015). This solar forcing configuration is a useful baseline against which data vs. model comparisons of thermospheric densities and temperatures can be conducted with a view to determining the role played by solar forcing (only) in the maintenance and variability (e.g. seasonal and solar cycle trends) of NGIMS measured fields. Thusfar, these MAVEN Deep Dip measurements clearly indicate that the thermospheric neutral densities and temperatures also vary significantly from orbit-to-orbit (~4-5 hours). However, solar forcing does not appear to control these inter-orbital variations. Wave activity from the lower atmosphere may be responsible.