Microscopic nonthermal plasma motions of coronal loops in a solar active region

We present a spectroscopic observation of a solar active region NOAA 7590 w ith a coronagraph at the Norikura Solar Observatory, which provides high-re solution spectra of the visible coronal emission lines (Fe x lambda 6374, F e XIV lambda 5303, Ca XV lambda 5694) with a spatial sampling of 2." 0 x 2. " 3. Nonthermal velocities (xi) estimated from Fe x lambda 6374, Fe XIV lam bda 5303, and Ca XV lambda 5694 in this observation are 14-20, 10-18, and 1 6-26 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 t he 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 Alfven waves is tested by caref ully 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 i s 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 ed ge-on loop appears to become smaller near the loop top than that for the fa ce-on loops. The obvious decrease of FWHM of 0.04-0.07 Angstrom (Delta xi = 3-5 km s(-1)) is found in the Fe XIV edge-on loops. Although this may be e vidence for the Alfven waves in coronal loops, the velocity amplitude seems to be too small to explain all the nonthermal velocity reported so far.