Measurements of Non-Thermal Line Widths in Solar Active Regions

TL;DR

This study measures non-thermal line widths in solar active regions using Hinode/EIS data, finding small velocities that challenge existing models of coronal heating and highlighting measurement difficulties.

Contribution

It provides the first comprehensive survey of non-thermal velocities in high-temperature solar active region loops, with results that question current theoretical models.

Findings

Mean non-thermal velocity of 17 km/s

No significant trend with temperature or magnetic flux

Results challenge reconnection and wave turbulence models

Abstract

Spectral line widths are often observed to be larger than can be accounted for by thermal and instrumental broadening alone. This excess broadening is a key observational constraint for both nanoflare and wave dissipation models of coronal heating. Here we present a survey of non-thermal velocities measured in the high temperature loops (1--5MK) often found in the cores of solar active regions. This survey of $\textit{Hinode}$ Extreme Ultraviolet Imaging Spectrometer (EIS) observations covers 15 non-flaring active regions that span a wide range of solar conditions. We find relatively small non-thermal velocities, with a mean value of 17km s$^{-1}$, and no significant trend with temperature or active region magnetic flux. These measurements appear to be inconsistent with those expected from reconnection jets in the corona, chromospheric evaporation induced by coronal nanoflares, and Alfv\'en wave turbulence models. Furthermore, because the observed non-thermal widths are generally small their measurements are difficult and susceptible to systematic effects.