Variation of Emission Line Width in Mid- and High-Latitude Corona

TL;DR

This study investigates how emission line widths vary with height in mid- and high-latitude solar corona regions, confirming previous equatorial findings and highlighting regional differences that may relate to solar wind acceleration.

Contribution

It extends prior research by analyzing multiple emission lines across different latitudes, revealing consistent line width variations and regional differences in the solar corona.

Findings

Red line FWHM increases with height

Green line FWHM decreases with height

Polar regions show higher line widths for most lines

Abstract

Spectroscopic studies of the solar corona, using the high spatial and spectral resolution 25-cm coronagraph at the Norikura observatory for equatorial off-limb observations, indicated that the variation of radiance and line width with height is different for different temperature lines. The line width of the forbidden red emission line [\FeX] 6374 \AA was found to increase with height and that of the green emission line [\FeXIV] 5303 \AA decrease with height. This had been interpreted in terms of the interaction between different temperature plasma but needed to be confirmed. Further observations were made on several days during 2004, in two emission lines simultaneously covering the mid-latitude and polar regions to investigate the existence of the observed variation in other parts of the solar corona. In this study, we have analysed several raster scans that cover mid- and high-latitude regions of the off-limb corona in all four bright emission lines [\FeX] 6374 \AA, [\FeXI] 7892 \AA, [\FeXIII] 10747 \AA, and [\FeXIV] 5303 \AA. We find that the FWHM of the red line increases with height and that of the green line decreases with height similar to that observed in equatorial regions. The line widths are found to be higher in polar regions for all of the observed emission lines except for the green line. Higher values of FWHM in polar regions may imply higher non-thermal velocities which could be further linked to a non-thermal source powering the solar-wind acceleration, but the reason for the behaviour of the green emission line remains to be explored.