Hinode EIS line widths in the quiet corona up to 1.5 R_⊙

¹ DAMTP, Centre for Mathematical Sciences, University of Cambridge, Wilberforce Road, Cambridge CB3 0WA, UK
e-mail: g.del-zanna@damtp.cam.ac.uk
² Udaipur Solar Observatory, Physical Research Laboratory, Badi Road, Udaipur 313 001, India

Received: 12 November 2018
Accepted: 29 September 2019

Abstract

We present an analysis of several Hinode Extreme-ultraviolet Imaging Spectrometer (EIS) observations of coronal line widths in the quiet Sun, up to 1.5 R_⊙ radial distances. Significant variations are found, which indicates no damping of Alfvén waves in the quiescent corona. However, the uncertainties in estimating the instrumental width mean that a firm conclusion cannot be reached. We present a discussion of various EIS instrumental issues and suggest that the strongest lines, from Fe XII at 193.5 Å and 195.1 Å, have anomalous instrumental widths. We also show how line widths in EIS are uncertain when the signal is low, and that the instrumental variation along the slit is also uncertain. We also find an anomalous decrease (up to 40%) in the intensities of these lines in many off-limb and active region observations, and provide evidence that this is due to opacity effects. We find that the most reliable measurements are obtained from the weaker lines.