The North-South Asymmetry of Martian Ionosphere and Thermosphere

Based on Mars Atmospheric and Volatile EvolutioN's neutral composition and Langmuir probe electron density and temperature data, we statistically analyzed the climatic variations of the Martian thermosphere and ionosphere and found significant north-south asymmetry. In winter and summer, the asymmetry mainly originates from the north-south asymmetry of the solar zenith angle. However, the observed data still exhibit significant north-south asymmetry in equinox seasons. Under low solar extreme ultraviolet radiation, the thermospheric neutral density in the northern hemisphere is higher than that in the southern hemisphere. As solar radiation increases, the neutral density in the southern hemisphere gradually exceeds that in the northern hemisphere. In addition, the southern hemisphere density increases nonlinearly with the increase of solar radiation, whereas the northern hemisphere density increases linearly. The electron density and electron temperature in the Martian ionosphere also exhibit similar north-south asymmetry. The asymmetries in the ionosphere and thermosphere between the northern and southern hemispheres are likely related to significant differences in Mars' north-south topography or to north-south asymmetries in the residual magnetic field. After preliminary analysis, we found that the north-south asymmetry of Mars' remaining magnetic field would intensify the hemispheric asymmetry of the ionospheric electron density, but have no effect on the thermospheric neutral density. The hemispheric asymmetry may be primarily related to the significant difference in Mars' north-south topography.