Magnetised turbulent plasmas as high-energy particle accelerators

TL;DR

The paper addresses how stochastic acceleration operates in magnetised turbulent plasmas, especially in large-amplitude, relativistic turbulence. It combines fully kinetic PIC results with a non-perturbative, covariant generalised Fermi framework to account for spatially intermittent, curvature-driven energisation and compression-related channels, leading to a broad, non-Gaussian distribution of acceleration rates. The key finding is that PIC spectra are power laws produced by an inhomogeneous acceleration landscape, not captured by a single diffusion coefficient; radiative losses and long-term feedback further shape the spectrum into near-universal high-energy behavior, potentially explaining extreme accelerators in astrophysical sources. The work provides a framework for predicting particle transport and energy spectra in VHE sources, with implications for blazars, pulsar wind nebulae, and SMBH coronae, and highlights open questions about low-amplitude turbulence and inertial-range physics.

Abstract

This proceedings paper reports on the theoretical modelling of particle acceleration in magnetised turbulent plasmas. It briefly reviews some recent findings obtained from fully kinetic numerical simulations of large-amplitude, semi to fully relativistic turbulence. The paper then argues that these findings can be understood within the framework of a ``generalised Fermi'' picture of stochastic acceleration, which it summarises. The dominant contributions to acceleration appear to arise from particle interactions with sharp, dynamic bends of the magnetic field lines and regions of velocity compression. Interestingly, the acceleration rate is spatially inhomogeneous and its probability distribution follows a broken power law extending up to large values. This makes relativistic, large-amplitude turbulence an extreme particle accelerator. Some implications for particle transport and the shape of the particle energy spectrum in the presence of radiative losses and over long timescales are also discussed.

Figures (2)

• Figure 1: Illustration of a PIC numerical simulations of relativistic, collisionless, large-amplitude, driven magnetised turbulence ($\delta B_{\rm rms}/B \sim 2$, $\sigma \sim 10$). Left panel: 3D view of a simulation cube, showing the plasma velocity in units of $c$. Right panel: particle energy distribution (thick red line); the dotted black line represents the prediction from a purely diffusive Fokker-Planck model. See text for details. Adapted from 2022PhRvD.106b3028B.
• Figure 2: Sketches illustrating the mechanisms of particle energisation in a generalised Fermi scenario, for particles interacting with a perturbation of extent $l\lesssim \lambda_{\rm s}$, where $\lambda_{\rm s}$ represents the particle mean free path to scattering in the turbulence. From left to right: curvature drift, corresponding to the variation of $\boldsymbol{u_E}$ along the magnetic field line; perpendicular compression of $\boldsymbol{u_E}$, encompassing gradient-drift and betatron acceleration. The third channel is not depicted; it involves the acceleration of the field line projected onto the magnetic field direction.