Spatially-Resolved Nonthermal Line Broadening During The Impulsive Phase of a Solar Flare

TL;DR

This study investigates nonthermal line broadening during a solar flare's impulsive phase, revealing correlations with plasma flows and densities, and suggesting turbulence and density effects as causes.

Contribution

It provides spatially-resolved analysis linking nonthermal broadening to plasma flows and densities during a solar flare, using Hinode/EIS and RHESSI data.

Findings

Strong correlation between nonthermal velocity and Doppler velocity at one footpoint.

No significant increase in electron density at the footpoints.

Pixels with higher nonthermal velocities are associated with higher densities.

Abstract

This paper presents a detailed study of excess line broadening in EUV emission lines during the impulsive phase of a C-class solar flare. In this work, which utilizes data from the EUV Imaging Spectrometer (EIS) onboard Hinode, the broadened line profiles were observed to be co-spatial with the two HXR footpoints as observed by RHESSI. By plotting the derived nonthermal velocity for each pixel within the Fe XV and Fe XVI rasters against its corresponding Doppler velocity a strong correlation (|r| > 0.59) was found between the two parameters for one of the footpoints. This suggested that the excess broadening at these temperatures is due to a superposition of flows (turbulence), presumably as a result of chromospheric evaporation due to nonthermal electrons. Also presented are diagnostics of electron densities using five pairs of density-sensitive line ratios. Density maps derived using the Mg XII and Si X line pairs showed no appreciable increase in electron density at the footpoints, while the Fe XII, Fe XIII, and Fe XIV line pairs revealed densities approaching 10^(11.5) cm^(-3). Using this information, the nonthermal velocities derived from the widths of the two Fe XIV lines were plotted against their corresponding density values derived from their ratio. This showed that pixels with large nonthermal velocities were associated with pixels of moderately higher densities. This suggests that nonthermal broadening at these temperatures may have been due to enhanced densities at the footpoints, although estimates of the amount of opacity broadening and pressure broadening appeared to be negligible.