An Introduction to Particle Acceleration in Shearing Flows

Abstract

<jats:p>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.</jats:p>

Particle Acceleration in Shearing Flows: Efficiencies and Limits

“…[4]) efficient electron acceleration typically requires the injection of energetic seed particles. The latter could, however, most likely be provided by conventional Fermi-type acceleration processes (Liu et al 2017;Rieger 2019). On the other hand, given their larger mean free path, shear acceleration of hadronic cosmic rays (CRs) is usually much easier to achieve.…”

“…Shear flows can in principle facilitate particle acceleration by several means (see Rieger 2019, for recent review). One prominent possibility includes a stochastic Fermi-type mechanism, in which particle energization occurs as a result of elastically scattering off differentially moving (magnetic) inhomogeneities (e.g., Berezhko & Krymskii 1981;Earl et al 1988;Webb 1989;Rieger & Duffy 2006;Lemoine 2019).…”

UHE Cosmic Rays And Agn Jets

“…The jets in AGN are likely to exhibit some internal jet stratification and velocity shearing, that could be conducive to efficient cosmic-ray acceleration, see ref. [35] for review and discussion. Prominent scenarios include Fermi-type particle acceleration in non-gradual (discontinuous), e.g.…”

Generalized Fermi Acceleration

“…The Ricci rotation coefficients Γâ bĉ characterize the space-time dependence of the vierbein, 4) and the symbol ∇ γ (written in short as ;γ in the following) represents a covariant derivative with respect to the metric g µν (x).…”

“…One commonly distinguishes various schemes of Fermi acceleration, e.g. stochastic (Fermi-II) acceleration in turbulence [2], diffusive shock (Fermi-I) acceleration [3], shear acceleration [4] etc., but, in truth, they only differ one from the other in how the velocity field is distributed throughout space. In order to describe the physics of acceleration, and more specifically, to evaluate the acceleration timescale t acc which is widely used in phenomenological applications, various methods have been proposed in the literature, with quite often a specific method applied to a given setting.…”