Observation of a Large Amplitude Slow Magnetosonic Wave in the Magnetosheath.

We report observations from the Magnetospheric Multiscale (MMS) satellites of pressure-balanced structures existing in a form of slow magnetosonic waves rather than the mirror mode description in the Earth's magnetosheath on Jan. 2, 2017. The slow magnetosonic waves are identified with two criteria: (1) The significant inverse correlation coefficient between local thermal pressure and magnetic pressure with a constant total pressure; (2) The observed dispersion relations match well with the theory of slow magnetosonic waves. It is found that the Landau damping rate in the magnetosheath plasma is much less than the mode frequency, which allows the mode to propagate in many periods with negligible decay. Furthermore, this work proposes that a high-speed ion beam resonating with the local ion acoustic mode contributes to energy transmissions between particles and waves.