Alfven Wave Turbulence and Perpendicular Ion Temperatures in Coronal Holes

TL;DR

This paper derives an analytic model for perpendicular ion temperatures in coronal holes driven by Alfven wave turbulence, successfully matching UVCS observations and explaining ion heating features.

Contribution

It introduces an analytic formula for stochastic ion heating in coronal holes, linking turbulence properties to observed ion temperature profiles.

Findings

Good fit to UVCS proton and OVI ion temperature data between 2-3 solar radii.

Explains preferential and anisotropic ion heating in coronal holes.

Accounts for rapid temperature increase and profile flattening of OVI ions.

Abstract

Low-frequency Alfven-wave turbulence causes ion trajectories to become chaotic, or "stochastic," when the turbulence amplitude is sufficiently large. Stochastic orbits enable ions to absorb energy from the turbulence, increasing the perpendicular ion temperature even when the fluctuation frequencies are too small for a cyclotron resonance to occur. In this paper, an analytic expression for the stochastic heating rate is used in conjunction with an observationally constrained turbulence model to obtain an analytic formula for the perpendicular ion temperature as a function of heliocentric distance r, ion mass, and ion charge in coronal holes for values of r between 2 and 15 solar radii (Rs). The resulting temperature profiles provide a good fit to observations of protons and OVI ions at 2Rs < r < 3Rs from the Ultraviolet Coronagraph Spectrometer (UVCS). Stochastic heating also offers a natural explanation for several detailed features of the UVCS observations, including the preferential and anisotropic heating of minor ions, the rapid radial increase in the OVI temperature between 1.6Rs and 1.9Rs, and the abrupt flattening of the OVI temperature profile as r increases above 1.9Rs.