Stochastic Acceleration by Turbulence

TL;DR

This paper reviews stochastic acceleration by plasma turbulence, highlighting its efficiency in energizing particles in astrophysical environments and its role in various acceleration mechanisms, especially in solar energetic particles.

Contribution

It emphasizes the importance of turbulence-driven stochastic acceleration as a dominant and efficient process, and proposes a hybrid model combining turbulence and shock acceleration.

Findings

Turbulence-driven acceleration is active in most astrophysical scenarios.

It preferentially accelerates electrons over protons in non-relativistic plasmas.

Hybrid acceleration models explain observed signatures of energetic particles in solar flares.

Abstract

The subject of this paper is stochastic acceleration by plasma turbulence, a process akin to the original model proposed by Fermi. We review the relative merits of different acceleration models, in particular the so called first order Fermi acceleration by shocks and second order Fermi by stochastic processes, and point out that plasma waves or turbulence play an important role in all mechanisms of acceleration. Thus, stochastic acceleration by turbulence is active in most situations. We also show that it is the most efficient mechanism of acceleration of relatively cool non relativistic thermal background plasma particles. In addition, it can preferentially accelerate electrons relative to protons as is needed in many astrophysical radiating sources, where usually there are no indications of presence of shocks. We also point out that a hybrid acceleration mechanism consisting of initial acceleration by turbulence of background particles followed by a second stage acceleration by a shock has many attractive features. It is demonstrated that the above scenarios can account for many signatures of the accelerated electrons, protons and other ions, in particular $^3$He and $^4$He, seen directly as Solar Energetic Particles and through the radiation they produce in solar flares.