Particle acceleration in strong MHD turbulence

TL;DR

This paper investigates how particles gain energy in strong, intermittent MHD turbulence, emphasizing the role of velocity gradients and their impact on particle momentum distributions, with implications for astrophysical phenomena.

Contribution

It introduces a new framework connecting velocity gradient intermittency to particle acceleration, extending beyond quasilinear theory and supported by direct simulation measurements.

Findings

Velocity gradients follow power law distributions due to intermittency.

Particle momentum distributions develop extended power law tails.

The model aligns with recent kinetic simulation results.

Abstract

Nonthermal acceleration of particles in magnetohydrodynamic (MHD) turbulence plays a central role in a wide variety of astrophysical sites. This physics is addressed here in the context of a strong turbulence, composed of coherent structures rather than waves, beyond the realm of quasilinear theory. The present description tracks the momentum of the particle through a sequence of frames in which the electric field vanishes, in the spirit of the original Fermi scenario. It connects the sources of energy gain (or loss) to the gradients of the velocity of the magnetic field lines, in particular the acceleration and the shear of their velocity flow projected along the field line direction, as well as their compression in the transverse plane. Those velocity gradients are subject to strong intermittency: they are spatially localized and their strengths obey power law distributions, as demonstrated through direct measurements in the incompressible MHD simulation of the Johns Hopkins University database. This intermittency impacts the acceleration process in a significant way, which opens up prospects for a rich phenomenology. In particular, the momentum distribution, which is here captured through an analytical random walk model, displays extended power law tails with soft-to-hard evolution in time, in general agreement with recent kinetic numerical simulations. Extensions to this description and possible avenues of exploration are discussed.