Microscopic Nonthermal Plasma Motions of Coronal Loops in a Solar Active Region

H. Hara, K. Ichimoto
1999 Astrophysical Journal  
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
more » ... 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.
doi:10.1086/306880 fatcat:mwphl53o7fbbnmqkxrv6k5tvru