On the Growth of Mirror Mode Waves in the Magnetosheath Based on Cluster Observations

Abstract

The evolution of mirror mode waves was investigated in the magnetosheath based on Cluster magnetic field and plasma measurements during four inbound passes in January-February 2006 when the separation between the spacecraft was about 10,000 km. The growth rates estimated from the amplitudes of the field fluctuations (mainly quasi-sinusoidal oscillations and peaks) observed simultaneously at distant locations were usually smaller than the maximum growth rates calculated from the plasma parameters measured in the magnetosheath suggesting that mirror mode waves do not grow beyond a certain degree, they seem to get saturated. Sudden changes in interplanetary conditions influenced the evolution of mirror mode waves, which were observed when solar wind dynamic pressure and the IMF were relatively quiet and the bow shock region connected to the observation point was quasi-perpendicular, while temperature anisotropy was moderate and ion β was high (1.2 <{ T}_{perp }/{T}_{\vert \vert } < 1.6 and {β }_i > 1 ) in the local plasma.