The Magnetopause Deformation Indicated by Fast Cold Ion Motion

The magnetopause deformation due to the upstream magnetosheath pressure perturbations is important to understand the solar wind-magnetosphere coupling process, but how to identify such events from in situ spacecraft observations is still challenging. In this study, we investigate magnetopause crossing events with fast-moving cold ions in the magnetosphere from Magnetospheric Multiscale observations, and find when fast-moving cold ions are present at the magnetopause, they are closely associated with the magnetopause deformation, which is featured by fast magnetopause motion and significant magnetopause normal deflection from model predictions. Therefore, fast-moving cold ions can be a useful indicator to search for magnetopause deformation events. By integrating the cold ion speed, the inferred magnetopause deformation amplitude varies from 0.1 to 2.0 RE. Further statistics indicate that such magnetopause deformation events prefer to occur under quasi-radial interplanetary magnetic field and fast solar wind conditions, suggesting high-speed magnetosheath jets could be one direct cause of magnetopause deformations. The magnetopause is found to be significantly deformed with the presence of fast-moving cold ions, whose speed can exceed 400 km s-1 The inferred magnetopause deformation amplitude varies from 0.1 to 2.0 RE The observed magnetopause deformation prefers to occur under quasi-radial interplanetary magnetic field and fast solar wind conditions