MMS direct observations of kinetic-scale shock self-reformation

TL;DR

This study provides direct observational evidence of shock self-reformation at a collisionless shock using MMS data, revealing how reflected ions form a new shock ramp and contribute to energy dissipation and particle acceleration.

Contribution

First direct observation of shock self-reformation process at a collisionless shock, illustrating ion dynamics and shock evolution in space plasma.

Findings

Reflected ions form a phase-space vortex leading to a new shock ramp

The new ramp is not yet fully developed and does not reflect ions strongly

Results support the role of self-reformation in energy dissipation and particle acceleration

Abstract

Studies of shocks have long suggested that a shock can undergo cyclically self-reformation in a time scale of ion cyclotron period. This process has been proposed as a primary mechanism for energy dissipation and energetic particle acceleration at shocks. Unambiguous observational evidence, however, has remained elusive. Here, we report direct observations for the self-reformation process of a collisionless, high Mach number, quasi-perpendicular shock using MMS measurements. We find that reflected ions by the old shock ramp form a clear phase-space vortex, which gives rise to a new ramp. The new ramp observed by MMS2 has not yet developed to a mature stage during the self-reformation, and is not strong enough to reflect incident ions. Consequently, these ions are only slightly slowed down and show a flat velocity profile from the new ramp all the way to the old one. The present results provide direct evidence for shock self-reformation, and also shed light on energy dissipation and energetic particle acceleration at collisionless shocks throughout the universe.