Statistical Study of Relations Between the Induced Magnetosphere, Ion Composition, and Pressure Balance Boundaries Around Mars Based On MAVEN Observations

Direct interaction between the solar wind (SW) and the Martian upper atmosphere forms a characteristic region, called the induced magnetosphere between the magnetosheath and the ionosphere. Since the SW deceleration due to increasing mass loading by heavy ions plays an important role in the induced magnetosphere formation, the ion composition is also expected to change around the induced magnetosphere boundary (IMB). Here we report on relations of the IMB, the ion composition boundary (ICB), and the pressure balance boundary based on a statistical analysis of about 8months of simultaneous ion, electron, and magnetic field observations by Mars Atmosphere and Volatile EvolutioN (MAVEN) mission. We chose the period when MAVEN observed the SW directly near its apoapsis to investigate their dependence on SW parameters. Results show that IMBs almost coincide with ICBs on the dayside and locations of all three boundaries are affected by the SW dynamic pressure. A remarkable feature is that all boundaries tend to locate at higher altitudes in the southern hemisphere than in the northern hemisphere on the nightside. This clear geographical asymmetry is permanently seen regardless of locations of the strong crustal B fields in the southern hemisphere, while the boundary locations become higher when the crustal B fields locate on the dayside. On the nightside, IMBs usually locate at higher altitude than ICBs. However, ICBs are likely to be located above IMBs in the nightside, southern, and downward E-SW hemisphere when the strong crustal B fields locate on the dayside.