Observations of plasma depletion in the magnetosheath at the dayside magnetopause

Abstract

In a set of 17 low‐ to mid‐latitude crossings of the dayside and near‐dayside magnetopause, Imp 6 plasma measurements show 11 cases of decreases in magnetosheath density just outside the boundary which are consistent with plasma depletion owing to magnetic flux tube compression as the field becomes draped against the magnetopause. Pressure anisotropies in the sense p⊥ >p∥ are a predicted result of the plasma depletion and field compression, and such anisotropies are observed. Application of the mirror instability criterion, which predicts growth of slow mode magnetoacoustic waves for values of p⊥/p∥ greater than a critical value, suggests that dayside magnetosheath plasma is usually unstable. One of the seventeen cases shows long‐period waveforms in the 100‐s density data that are 180° out of phase with simultaneous waveforms in the magnetic field strength. We interpret these data as the signature of slow mode magnetoacoustic waves.