Ion acceleration to MeV by the ExB wave mechanism in collisionless shocks

TL;DR

This paper demonstrates that ions can be accelerated to MeV energies via the ExB wave mechanism in collisionless shocks, explaining observed ion energization in space and astrophysical shock environments.

Contribution

It introduces a new model for ion acceleration to MeV energies through the ExB wave mechanism, linking it to specific plasma instabilities and providing a general maximum energy expression.

Findings

Ions can reach MeV energies perpendicular to magnetic fields.

The acceleration occurs in discrete steps related to the gyroperiod.

The mechanism explains observations in space and astrophysical shocks.

Abstract

It is shown that ions can be accelerated to MeV energy range in the direction perpendicular to the magnetic field by the ExB mechanism of electrostatic waves. The acceleration occurs in discrete steps of duration being a small fraction the gyroperiod and can explain observations of ion energization to 10 keV at quasi-perpendicular shocks and to 100-1000 keV at quasi-parallel shocks. A general expression is provided for the maximum energy of ions accelerated in shocks of arbitrary configuration. The waves involved in the acceleration are related to three cross-field current-driven instabilities: the lower hybrid drift (LHD) instability induced by the density gradients in shocks and shocklets, followed by the modified two-stream (MTS) and electron cyclotron drift (ECD) instabilities, induced by the ExB drift of electrons in the strong LHD wave electric field. The ExB wave mechanism accelerates heavy ions to energies proportional to the atomic mass number, which is consistent with satellite observations upstream of the bow shock and also with observations of post-shocks in supernovae remnants.