MAVEN observations on a hemispheric asymmetry of precipitating ions toward the Martian upper atmosphere according to the upstream solar wind electric field

摘要

The Mars Atmosphere and Volatile Evolution (MAVEN) observations show that the global spatial distribution of ions precipitating toward the Martian upper atmosphere has a highly asymmetric pattern relative to the upstream solar wind electric field. MAVEN observations indicate that precipitating planetary heavy ion fluxes measured in the downward solar wind electric field (-E) hemisphere are generally larger than those measured in the upward electric field (+E) hemisphere, as expected from modeling. The -E (+E) hemispheres are defined by the direction of solar wind electric field pointing toward (or away from) the planet. On the other hand, such an asymmetric precipitating pattern relative to the solar wind electric field is less clear around the terminator. Strong precipitating fluxes are sometimes found even in the +E field hemisphere under either strong upstream solar wind dynamic pressure or strong interplanetary magnetic field periods. The results imply that those intense precipitating ion fluxes are observed when the gyroradii of pickup ions are estimated to be relatively small compared with the planetary scale. Therefore, the upstream solar wind parameters are important factors in controlling the global spatial pattern and flux of ions precipitating into the Martian upper atmosphere.