Solar off-limb line widths: Alfven waves, ion-cyclotron waves, and preferential heating

TL;DR

This study develops a method to analyze off-limb solar spectra to constrain Alfven wave amplitudes and ion-cyclotron heating, revealing preferential heating of ions with small charge-to-mass ratios in the solar corona.

Contribution

It introduces a new approach to separate non-thermal and thermal line widths in coronal spectra, providing evidence for ion-cyclotron preferential heating.

Findings

Stray light explains the apparent decrease in spectral line widths with height.

Ions with smaller charge-to-mass ratios are hotter and more strongly heated.

No direct evidence of Alfven wave damping was observed.

Abstract

Alfven waves and ion-cyclotron absorption of high-frequency waves are frequently brought into models devoted to coronal heating and fast solar-wind acceleration. Signatures of ion-cyclotron resonance have already been observed in situ in the solar wind (HELIOS spacecrafts) and, recently, in the upper corona (UVCS/SOHO remote-sensing results). We propose a method to constrain both the Alfven wave amplitude and the preferential heating induced by ion-cyclotron resonance, above a partially developed polar coronal hole observed with the SUMER/SOHO spectrometer. The instrumental stray light contribution is first substracted from the spectra. By supposing that the non-thermal velocity is related to the Alfven wave amplitude, it is constrained through a density diagnostic and the gradient of the width of the Mg X 625 A line. The temperatures of several coronal ions, as functions of the distance above the limb, are then determined by substracting the non-thermal component to the observed line widths. The effect of stray light explains the apparent decrease with height in the width of several spectral lines, this decrease usually starting about 0.1-0.2 Rs above the limb. This result rules out any direct evidence of damping of the Alfven waves, often suggested by other authors. We also find that the ions with the smallest charge-to-mass ratios are the hottest ones at a fixed altitude and that they are subject to a stronger heating, as compared to the others, between 57" and 102" above the limb. This constitutes a serious clue to ion-cyclotron preferential heating.