Fe XII and Fe XIII Line Widths in the Polar Off-limb Solar Corona up to 1.5 Rsun

TL;DR

This study measures and analyzes the line widths of Fe XII and Fe XIII in the solar corona up to 1.5 solar radii, comparing observations with models to understand nonthermal broadening and the influence of streamers.

Contribution

It provides detailed measurements of spectral line widths in the off-limb solar corona and compares them with model predictions, highlighting discrepancies and the impact of streamers.

Findings

Line widths increase below 1.1 R_sun and fluctuate afterward.

Synthetic line widths are lower than observed below 1.3 R_sun.

Streamer emission significantly contaminates coronal hole line profiles.

Abstract

The nonthermal broadening of spectral lines formed in the solar corona is often used to seek the evidence of Alfv\'en waves propagating in the corona. To have a better understanding of the variation of line widths at different altitudes, we measured the line widths of the strong Fe \textsc{xii} 192.4, 193.5, and 195.1 \mbox{\AA} and Fe \textsc{xiii} 202.0 \mbox{\AA} in an off-limb southern coronal hole up to 1.5 $R_\odot$ observed by the Extreme Ultraviolet Spectrometer (EIS) on board the \textit{Hinode} satellite. We compared our measurements to the predictions from the Alfv\'en Wave Solar Model (AWSoM) and the SPECTRUM module. We found that the Fe \textsc{xii} and Fe \textsc{xiii} line widths first increase monotonically below 1.1 $R_\odot$, and then keep fluctuating between 1.1 and 1.5 $R_\odot$. The synthetic line widths of Fe \textsc{xii} and Fe \textsc{xiii} below 1.3 $R_\odot$ are notably lower than the observed ones. We found that the emission from a streamer in the line of sight significantly contaminates the coronal hole line profiles even up to 1.5 $R_\odot$ both in observations and simulations. We suggest that either the discrepancy between the observations and simulations is caused by insufficient nonthermal broadening at the streamer in the AWSoM simulation or the observations are less affected by the streamer. Our results emphasize the importance of identifying the origin of the coronal EUV emission in off-limb observations.