Spatially resolved nonthermal line broadening during the impulsive phase of a solar flare

Abstract

This paper presents a detailed study of excess line broadening in extreme ultraviolet (EUV) emission lines during the impulsive phase of a C-class solar flare. In this work, which utilizes data from the EUV Imaging Spectrometer on board Hinode, the broadened line profiles were observed to be cospatial with the two hard X-ray footpoints as observed by RHESSI. By plotting the derived nonthermal velocity for each pixel within the FeXV and FeXVI 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 MgVII and SiX line pairs showed no appreciable increase in electron density at the footpoints, whereas the FeXII, FeXIII, and FeXIV line pairs revealed densities approaching 1011.5cm-3. Using this information, the nonthermal velocities derived from the widths of the two FeXIV 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. © 2011. The American Astronomical Society. All rights reserved.