Microscopic Nonthermal Plasma Motions of Coronal Loops in a Solar Active Region
We present a spectroscopic observation of a solar active region NOAA 7590 with a coronagraph at the Norikura Solar Observatory, which provides high-resolution spectra of the visible coronal emission lines (Fe X j6374, Fe XIV j5303, Ca XV j5694) with a spatial sampling of Nonthermal veloci-2A .0 ] 2A .3. ties (m) estimated from Fe X j6374, Fe XIV j5303, and Ca XV j5694 in this observation are 14È20, 10È18, and 16È26 km s~1, respectively. The Ðrst two results are consistent with the results
... ed by Cheng et al. and others. Even in the Ca XV structures the present observation does not conÐrm the large nonthermal velocity of 40È60 km s~1 to be typical value. The hypothesis that the nonthermal width in coronal emission lines is due to coronal waves is tested by carefully examining the relationship Alfve n between the width of coronal emission lines and orientation of coronal loops to the line-of-sight direction. From the comparison between edge-on loops in which the direction of magnetic Ðeld is nearly parallel to the line-of-sight direction and face-on loops in which the magnetic Ðeld is almost perpendicular to the line-of-sight direction, the full width at half-maximum (FWHM) of coronal emission lines for the edge-on loop appears to become smaller near the loop top than that for the face-on loops. The obvious decrease of FWHM of 0.04È0.07 (*m \ 3È5 km s~1) is found in the Fe XIV edge-on loops. A Although this may be evidence for the waves in coronal loops, the velocity amplitude seems to be Alfve n too small to explain all the nonthermal velocity reported so far.