Sunward O+ ion jets reflected below the Martian bow shock: MAVEN observations

Abstract:

We show existence of energetic quasi-sunward O⁺ ion jets (~10 keV or higher) reflected below the Martian bow shock by analyzing ion velocity distribution functions (VDFs) measured by the Superthermal and Thermal Ion Composition (STATIC) instrument on the Mars Atmosphere and Volatile Evolution missioN (MAVEN). In the solar wind near Mars, MAVEN often observes energetic quasi-sunward O⁺ ion jets. In the magnetosheath near the O⁺ ion jets event, a partial ring distribution is sometimes seen in the O⁺ ion VDFs. The partial ring distribution is not on a ring distribution corresponding to newly-born pickup ions but on another ring distribution corresponding to pickup ions with an initial velocity (i.e. not newly-born pickup ions). We interpret that the partial ring distribution is produced by the reflection of the precipitating O⁺ ions below the bow shock. After being injected into the magnetosheath and sometimes subsequently into the induced magnetosphere from the solar wind, the precipitating O⁺ ions are subject to the deflection by fields in these regions, and consequently, a part of precipitating O⁺ ions are reflected back to the solar wind with the incident energy. We find that the ion reflection preferably occurs near the subsolar region where the velocity of the shocked solar wind becomes slow (~100 km/s) and the magnetic field becomes strong (~20 nT), making the gyro radius of the precipitating O⁺ ions (> 10 keV) comparable to the spatial scale of the magnetosheath and the induced magnetosphere (~2000 km on the dayside). We discuss the importance of the O⁺ ion jets in terms of understanding the oxygen escape processes at Mars.