Abstract
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 (MMS) 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 0.2 to ∼ 2.5 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.