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

Abstract

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 λ6374, Fe XIV λ5303, Ca XV λ5694) with a spatial sampling of 20×23. Nonthermal velocities (ξ) estimated from Fe X λ6374, Fe XIV λ5303, and Ca XV λ5694 in this observation are 14-20, 10-18, and 16-26 km s^-1, respectively. The first two results are consistent with the results obtained by Cheng et al. and others. Even in the Ca XV structures the present observation does not confirm the large nonthermal velocity of 40-60 km s^-1 to be typical value. The hypothesis that the nonthermal width in coronal emission lines is due to coronal Alfvén waves is tested by carefully examining the relationship 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 field is nearly parallel to the line-of-sight direction and face-on loops in which the magnetic field 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 0.04-0.07 Å (Δξ=3-5 km s^-1) is found in the Fe XIV edge-on loops. Although this may be evidence for the Alfvén waves in coronal loops, the velocity amplitude seems to be too small to explain all the nonthermal velocity reported so far.