The Efficiency of Second-Order Fermi Acceleration by Weakly Compressible MHD Turbulence

TL;DR

This study examines how pitch-angle scattering influences particle heating and acceleration in MHD turbulence, finding that while it enhances resonant heating, it is unlikely to produce the observed power-law tails in solar wind proton distributions.

Contribution

It provides a detailed analysis of the effects of pitch-angle scattering on particle acceleration in MHD turbulence, highlighting the limitations in generating power-law tails in solar wind conditions.

Findings

Resonant interactions increase heating efficiency by factors of a few-10.

High scattering rates lead to non-resonant, adiabatic heating and cooling.

Power-law tails in proton distributions are unlikely from MHD turbulence in the solar wind.

Abstract

We investigate the effects of pitch-angle scattering on the efficiency of particle heating and acceleration by MHD turbulence using phenomenological estimates and simulations of non-relativistic test particles interacting with strong, subsonic MHD turbulence. We include an imposed pitch-angle scattering rate, which is meant to approximate the effects of high frequency plasma waves and/or velocity space instabilities. We focus on plasma parameters similar to those found in the near-Earth solar wind, though most of our results are more broadly applicable. An important control parameter is the size of the particle mean free path lambda_{mfp} relative to the scale of the turbulent fluctuations L. For small scattering rates, particles interact quasi-resonantly with turbulent fluctuations in magnetic field strength. Scattering increases the long-term efficiency of this resonant heating by factors of a few-10, but the distribution function does not develop a significant non-thermal power-law tail. For higher scattering rates, the interaction between particles and turbulent fluctuations becomes non-resonant, governed by particles heating and cooling adiabatically as they encounter turbulent density fluctuations. Rapid pitch-angle scattering can produce a power-law tail in the proton distribution function but this requires fine-tuning of parameters. Moreover, in the near-Earth solar wind, a significant power-law tail cannot develop by this mechanism because the particle acceleration timescales are longer than the adiabatic cooling timescale set by the expansion of the solar wind. Our results thus imply that MHD-scale turbulent fluctuations are unlikely to be the origin of the v^{-5} tail in the proton distribution function observed in the solar wind.