An Introduction to Particle Acceleration in Shearing Flows

TL;DR

This paper reviews how shear flows in space and astrophysical plasmas can accelerate charged particles through various mechanisms, potentially explaining the origin of high-energy cosmic rays and energetic electrons in astrophysical sources.

Contribution

It provides a comprehensive overview of recent developments in particle acceleration mechanisms within shear flows, highlighting processes beyond traditional shock acceleration.

Findings

Shear flows can efficiently accelerate particles via microscopic instabilities.

Fermi-type acceleration occurs in both gradual and non-gradual shear flows.

Large-scale turbulence also contributes to particle acceleration in shear environments.

Abstract

Shear flows are ubiquitously present in space and astrophysical plasmas. This paper highlights the central idea of the non-thermal acceleration of charged particles in shearing flows and reviews some of the recent developments. Topics include the acceleration of charged particles by microscopic instabilities in collisionless relativistic shear flows, Fermi-type particle acceleration in macroscopic, gradual and non-gradual shear flows, as well as shear particle acceleration by large-scale velocity turbulence. When put in the context of jetted astrophysical sources such as Active Galactic Nuclei, the results illustrate a variety of means beyond conventional diffusive shock acceleration by which power-law like particle distributions might be generated. This suggests that relativistic shear flows can account for efficient in-situ acceleration of energetic electrons and be of relevance for the production of extreme cosmic rays.