Quasilinear Consequences of Turbulent Ion Heating by Magnetic Moment Breaking

TL;DR

This paper investigates the quasilinear consequences of magnetic moment breaking (MMB) on ion heating in the solar wind, revealing that the instability limits anisotropy and enhances particle energization, impacting kinetic models of plasma heating.

Contribution

It introduces a model combining MMB dissipation with ion-cyclotron instability, showing how these processes influence proton distribution and energy in the solar wind.

Findings

MMB-induced anisotropies are limited to reasonable values.

The combined MMB and instability processes increase particle energization.

Proton distribution shapes differ from previous models.

Abstract

The fast solar wind emerging from coronal holes is likely heated and accelerated by the dissipation of magnetohydrodynamic turbulence, but the specific kinetic mechanism resulting in the perpendicular ion heating required by observations is not understood. A promising mechanism has been proposed by Chandran et al. (2010), which in this paper we call "magnetic moment breaking" (MMB). As currently formulated, MMB dissipation operates only on the ion perpendicular motion, and does not influence their parallel temperature. Thus, the MMB mechanism acting by itself produces coronal hole proton distributions that are unstable to the ion-cyclotron anisotropy instability. This quasilinear instability is expected to operate faster than the nonlinear turbulent cascade, scattering ions into the parallel direction and generating quasi-parallel-propagating ion-cyclotron (IC) waves. To investigate the consequences of this instability on the MMB-heated protons, we construct a homogeneous model for protons with coronal hole properties. Using a simplified version of the resonant cyclotron interaction, we heat the protons by the MMB process and instantaneously scatter them to lower anisotropy while self-consistently generating parallel-propagating IC waves. We present several illustrative cases, finding that the extreme anisotropies implied by the MMB mechanism are limited to reasonable values, but the distinctive shape of the proton distribution derived by Klein & Chandran (2016) is not maintained. We also find that these combined processes can result in somewhat higher particle energization than the MMB heating alone. These quasilinear consequences should follow from any kinetic mechanism that primarily increases the perpendicular ion temperature in a collisionless plasma.