Fine structure and motion of the bow shock and particle energisation mechanisms inferred from Magnetospheric Multiscale (MMS) observations

Abstract

This study presents new observations of fine structure and motion of the bow shock formed in the solar wind, upstream of the Earth's magnetosphere. NASA's Magnetospheric Multiscale (MMS) mission has recorded data during 11 encounters with a shock oscillating with frequency of 1 mHz. Shocks move with a speed of 4–17 km s⁻¹; have thickness of 100 km, i.e. an ion gyroradius; and represent cascades of compressional magnetic field and plasma density structures of increasing frequencies or smaller spatial scales. Induced density gradients initiate chains of cross-field current-driven instabilities that heat solar wind ions by the stochastic $\stackrel{\mathrm{̃}}{E}\times B$ wave energisation mechanism. The theoretical ion energisation limits are confirmed by observations. We have identified the ion acceleration mechanism operating at shocks and explained double-beam structures in the velocity space. The nature of this mechanism has been revealed as a stochastic resonant acceleration (SRA). The results provide for the first time a consistent picture of a chain of plasma processes that generate collisionless shocks and are responsible for particle energisation.