Martian ionosphere observed by MAVEN. 3. Influence of solar wind and IMF on upper ionosphere

Abstract

In two previous papers (Dubinin et al., 2016, 2017a) we studied the influence of the crustal magnetic field and solar irradiance on the upper ionosphere of Mars based on the observations made on Mars Express. Here we discuss effects of solar wind and the interplanetary magnetic field using the MAVEN observations. It is shown that the upper ionosphere at solar zenith angles ≥∼40o and altitudes above ∼350−400 km is asymmetrical along the direction of the motional electric field. In the hemisphere, in which the motional electric field is directed toward the planet, the ionosphere is denser and expands to higher altitudes as compared to the ionosphere in the opposite hemisphere. The difference in the density of O+ and O2+ ions in both hemispheres achieves almost one order. Such a difference arises due to a different ion convection. Ion convection in both ionospheres occurs in the same direction as the motion of the shocked solar wind in the adjacent magnetosheath although with very different velocities. The upper ionosphere is also sensitive to the variations in the solar wind dynamic pressure, solar wind flux and the value of the motional electric field with a distinct depletion at altitudes above ∼450−500 km with increase of these solar wind characteristics. The depletion is accompanied by an increase in the horizontal ion velocities. As a result, the ion fluxes to the tail only insignificantly vary with the modest variations of the solar wind characteristics.