Do the Line Widths of Coronal Emission Lines Increase with Height above the Limb?

Abstract

In our earlier studies we obtained off-the-limb spectroscopic observations in a number of forbidden emission lines ([Fe X-XIV]) to study the physical properties and their temporal variations in steady coronal structures. Short exposure times adopted in those observations permitted us to study the variation in line widths up to about 150'' above the limb. With a view to investigating the variations in the parameters of coronal emission lines up to about 500'', we made raster scans with exposure times that are longer than the earlier exposure times by a factor of about 10. We find that the FWHM of the [Fe XIV] 5303 Å line decreases up to 300'' ± 50'' and then remains more or less the same up to 500'', while that of the [Fe X] 6374 Å line increases up to about 250'' and subsequently remains unchanged. The FWHMs of the [Fe XI] 7892 Å and [Fe XIII] 10747 Å lines show an intermediate behavior. Furthermore, the ratio of the FWHM of 6374 to 5303 Å increases from 0.93 at the limb to 1.18 at 200'' above the limb. The nonvariability in the FWHM of emission lines after about 300'' above the limb in steady coronal structures does not support the prevailing view that the nonthermal velocity increases with height due to either the coronal waves or the high-velocity solar wind. The present results indicate the inadequacy of the earlier coronal loop models. The observed variations in FWHM of the coronal emission lines with height above the limb can be explained by assuming the recent model of coronal loops proposed by Akiyama et al.