Comparison of Mars Initial Reference Ionosphere (MIRI) Model with Initial Data Sets from MAVEN

Abstract:

Ionospheric structure and plasma escape play key roles in the evolution of the martian atmosphere. MAVEN’s Neutral Gas and Ion Mass Spectrometer (NGIMS) measured the distribution of thermal ions, and the Langmuir Probe and Waves (LPW) measured the electron density, at ionospheric heights (~130-400 km) during the satellite’s “Deep-Dip” orbit campaign between April 17 and 22, 2015. These data included mid-day orbit segments (LT ≈ 11-13, SZA ≈ 0^o- 45^o) where ion compositions were observed for the first time. We used the sum of ion densities as a proxy for electron density and compared results with a new version of the Mars Initial Reference Ionosphere (MIRI). At altitudes where the transition occurs between photo-chemical and dynamical processes (130-200 km), the NGIMS results agreed with the shape of the MIRI N_e(h) predictions at all heights and solar zenith angles encountered, but the model predictions are a factor of two higher in absolute magnitude. For altitudes above 200 km, the MAVEN NGIMS patterns diverge considerably from MIRI predictions, for reasons that may involve crustal-B field effects, ionopause-like boundaries, and horizontal plasma transport away from the noon section not yet included in MIRI. Calibration uncertainties of both data and model might also contribute to these differences. For example, subsequently available LPW observations support N_e(h) values closer to the MIRI predictions.