Martian Ionosphere-Thermosphere Coupling in Longitude Structures: Statistical Results for the Main Ionization Peak Height

Abstract

The Martian ionosphere-thermosphere (I-T) coupling is variable due to complex variations of the driving factors such as atmospheric tides and crustal magnetic fields. In this study, variability of the I-T coupling in longitude structures was investigated using a series of data segments of the MGS ionospheric measurements. Measurements in each data segment can cover different longitudes, and the solar forcing and local solar time just change a little. Ionospheric and thermospheric longitude variations are statistically correlated. Ionospheric peak electron density (N_mM₂) decreases while ionospheric main peak height (h_mM₂) increases with increasing neutral scale height (H_n) along longitudes. These correlated longitude variations are consistent with the photochemical coupling that H_n longitude disturbances induce ionospheric longitude structure through photochemical processes. Statistically, N_mM₂ is a better indicator than h_mM₂ for the H_n disturbances in the lower thermosphere. H_n longitude variation intensity is a crucial factor affecting the photochemical I-T coupling in longitude structures; it is closely related to N_mM₂ longitude variation intensity and tends to decline with increasing altitudes. The I-T coupling in longitude structures tends to decline near the terminator, which is in line with the declining longitude variation of H_n with increasing altitudes since h_mM₂ significantly increases near the terminator. Moreover, it tends to enhance at high solar activity level due to increased photoionization rate. The I-T coupling in longitude structures also shows seasonal dependence, as seasonal variation of h_mM₂ can affect the H_n longitude variation intensity nearby the ionospheric main peak.