Ion Acceleration at the Quasi-Parallel Bow Shock: Decoding the Signature of Injection

TL;DR

This study uses spacecraft data and simulations to identify the initial ion reflection signatures at Earth's bow shock, revealing how ions gain energy and start the acceleration process in collisionless shocks.

Contribution

It uncovers the specific upstream wave boundary where ion reflection initiates injection, combining observational data with hybrid simulations.

Findings

Ion reflection signatures occur at upstream wave boundaries.

Reflected ions gain energy through multiple shock interactions.

Injection completes within three to five gyroperiods.

Abstract

Collisionless shocks are efficient particle accelerators. At Earth, ions with energies exceeding 100 keV are seen upstream of the bow shock when the magnetic geometry is quasi-parallel, and large-scale supernova remnant shocks can accelerate ions into cosmic rays energies. This energization is attributed to diffusive shock acceleration, however, for this process to become active the ions must first be sufficiently energized. How and where this initial acceleration takes place has been one of the key unresolved issues in shock acceleration theory. Using Cluster spacecraft observations, we study the signatures of ion reflection events in the turbulent transition layer upstream of the terrestrial bow shock, and with the support of a hybrid simulation of the shock, we show that these reflection signatures are characteristic of the first step in the ion injection process. These reflection events develop in particular in the region where the trailing edge of large-amplitude upstream waves intercept the local shock ramp and the upstream magnetic field changes from quasi-perpendicular to quasi-parallel. The dispersed ion velocity signature observed can be attributed to a rapid succession of ion reflections at this wave boundary. After the ions' initial interaction with the shock, they flow upstream along the quasi-parallel magnetic field. Each subsequent wave front in the upstream region will sweep the ions back toward the shock, where they gain energy with each transition between the upstream and the shock wave frames. Within three to five gyroperiods, some ions have gained enough parallel velocity to escape upstream, thus completing the injection process.