Estimating Ion Temperatures at the Polar Coronal Hole Boundary

TL;DR

This study measures ion temperatures at the polar coronal hole boundary, revealing preferential heating of heavy ions and exploring the effects of line broadening and plasma motions on temperature diagnostics.

Contribution

It provides new measurements of ion temperatures across a range of charge-to-mass ratios using coordinated satellite observations, improving understanding of coronal heating mechanisms.

Findings

Heavy ions with Z/A < 0.20 or > 0.33 have higher temperatures than electrons.

Plasma bulk motions along the line of sight broaden emission lines, affecting temperature estimates.

Non-Gaussian line profiles suggest complex velocity distributions in the coronal plasma.

Abstract

The temperatures of the heavy ions ($T_i$) in the solar corona provide critical information about the heating mechanism of the million-degree corona. However, the measurement of $T_i$ is usually challenging due to the nonthermal motion, instrumental limitations, and the optically thin nature of the coronal emissions. We present the measurement of $T_i$ and its dependency on the ion charge-to-mass ratio ($Z/A$) at the polar coronal hole boundary, only assuming that heavy ions have the same nonthermal velocity. To improve the $Z/A$ coverage and study the influence of the instrumental broadening, we used a coordinated observation from the extreme-ultraviolet Imaging Spectrometer (EIS) on board the Hinode satellite and the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) on board the Solar and Heliospheric Observatory (SOHO). We found that the $T_i$ of ions with $Z/A$ less than 0.20 or greater than 0.33 are much higher than the local electron temperature. We ran the Alfv\'en Wave Solar Model-realtime to investigate the formation of optically thin emissions along the line of sight (LOS). The simulation suggested that plasma bulk motions along the LOS broaden the widths of hot emission lines in the coronal hole (e.g., Fe XII, Fe XIII). We discussed other factors that might affect the $T_i$ measurement, including the non-Gaussian wings in some bright SUMER lines, which can be fitted by a double-Gaussian or a $\kappa$ distribution. Our study confirms the preferential heating of heavy ions in coronal holes and provides new constraints on coronal heating models.