Diffusive Shock Re-Acceleration

TL;DR

This study uses 2D hybrid simulations to explore how pre-existing energetic particles are re-accelerated at collisionless shocks, leading to magnetic field amplification and thermal particle injection, relevant for cosmic ray acceleration in various astrophysical environments.

Contribution

It introduces the concept of diffusive shock re-acceleration, demonstrating its effectiveness across shock inclinations and its role in magnetic field amplification and thermal particle injection.

Findings

Re-accelerated seeds drive streaming instability upstream.

Magnetic field amplification can reach non-linear levels in less than 10 years.

Re-acceleration can trigger thermal particle injection at oblique shocks.

Abstract

We have performed 2D hybrid simulations of non-relativistic collisionless shocks in the presence of pre-existing energetic particles ("seeds"); such a study applies, for instance, to the re-acceleration of Galactic cosmic rays (CRs) in supernova remnant (SNR) shocks and solar wind energetic particles in heliospheric shocks. Energetic particles can be effectively reflected and accelerated regardless of shock inclination via a process that we call \emph{diffusive shock re-acceleration}. We find that reaccelerated seeds can drive the streaming instability in the shock upstream and produce effective magnetic field amplification. This can eventually trigger the injection of thermal protons even at oblique shocks that ordinarily cannot inject thermal particles. We characterize the current in reflected seeds, finding that it tends to a universal value $J\simeq e n_{cr} v_{sh}$, where $e n_{cr}$ is the seed charge density and $v_{sh}$ is the shock velocity. When applying our results to SNRs, we find that the re-acceleration of Galactic CRs can excite the Bell instability to non-linear levels in less than $\sim 10$ yr, thereby providing a minimum level of magnetic field amplification for any SNR shock. Finally, we discuss the relevance of diffusive shock re-acceleration also for other environments, such as heliospheric shocks, Galactic superbubbles, and clusters of galaxies.