Proton, Electron, and Ion Heating in the Fast Solar Wind from Nonlinear Coupling Between Alfvenic and Fast-Mode Turbulence

TL;DR

This paper models collisionless plasma heating in the solar wind by analyzing nonlinear coupling between Alfvenic and fast-mode turbulence, predicting ion heating rates consistent with observations.

Contribution

It introduces a new model that incorporates nonlinear coupling between MHD wave modes to predict collisionless heating rates in the solar wind.

Findings

Predicted ion heating rates match observational data.

Fast-mode energy transfer excites ion cyclotron resonance.

High-frequency waves are a small part of the total spectrum.

Abstract

In the parts of the solar corona and solar wind that experience the fewest Coulomb collisions, the component proton, electron, and heavy ion populations are not in thermal equilibrium with one another. Observed differences in temperatures, outflow speeds, and velocity distribution anisotropies are useful constraints on proposed explanations for how the plasma is heated and accelerated. This paper presents new predictions of the rates of collisionless heating for each particle species, in which the energy input is assumed to come from magnetohydrodynamic (MHD) turbulence. We first created an empirical description of the radial evolution of Alfven, fast-mode, and slow-mode MHD waves. This model provides the total wave power in each mode as a function of distance along an expanding flux tube in the high-speed solar wind. Next we solved a set of cascade advection-diffusion equations that give the time-steady wavenumber spectra at each distance. An approximate term for nonlinear coupling between the Alfven and fast-mode fluctuations is included. For reasonable choices of the parameters, our model contains enough energy transfer from the fast mode to the Alfven mode to excite the high-frequency ion cyclotron resonance. This resonance is efficient at heating protons and other ions in the direction perpendicular to the background magnetic field, and our model predicts heating rates for these species that agree well with both spectroscopic and in situ measurements. Nonetheless, the high-frequency waves comprise only a small part of the total Alfvenic fluctuation spectrum, which remains highly two-dimensional as is observed in interplanetary space.