Reflection of precipitating O+ pickup ions in the Martian magnetosheath observed by MAVEN
Kei Masunaga
Swedish Institute of Space Physics
Abstract
Due to no intrinsic magnetic field at Mars, the solar wind directly interacts with the Martian upper atmosphere. The solar wind convective electric field accelerates (or “pick-upâ€) planetary ions such as O+ ions in the upper atmosphere, which allows them to escape Mars by forming an ion plume. Since gyroradii of the pickup ions around Mars are comparable to the Martian radius or even larger, some pickup ions precipitate towards Mars.
In this study, we analyze O+ ion velocity distribution functions (VDFs) obtained from the Suprathermal and Thermal Ion Composition (STATIC) instrument on the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft, and identify reflections of the precipitating O+ pickup ions. In the Martian magnetosheath, MAVEN observes characteristic O+ ion VDFs: a partial ring distribution. The partial ring distribution corresponds to pickup ions with a finite initial velocity (i.e., not newborn pickup ions), and its phase space density is much smaller than that of local pickup O+ ions of the magnetosheath. This suggests that after being injected into the magnetosheath from the solar wind, the precipitating O+ ions are subject to the significantly enhanced magnetic field in this region and start to gyrate around the guiding center of the plasma frame in the magnetosheath. Consequently, a part of precipitating O+ ions are reflected back to the solar wind, generating O+ beams in the solar wind.
We also quantitatively study the ion reflection feature. Analyzing ~1 year data set of STATIC, we find that the average ion reflection ratio is ~14 %. We also study its solar wind dependence, and find that the reflection ratio goes up to ~40 % with the magnitude of the interplanetary magnetic field. Since the precipitating O+ ions are known to be a major source of the sputtering escape at Mars, this result implies that the ion reflection may have a role to reduce the sputtering effects at Mars under the extreme solar wind condition.
Created 2017-04-18 12:30:45 by Mats Holmström Last changed 2017-04-18 12:30:45 by Mats Holmström
