Thermal structure of coronal loops as seen with Norikura coronagraph

Abstract

The thermal structure of a coronal loop, both along and across the loop, is vital in determining the exact plasma heating mechanism. High-resolution spectroscopic observations of the off-limb corona were made using the 25 cm Norikura coronagraph, located at Norikura, Japan. Observations on a number of days were made simultaneously in four forbidden iron emission lines, namely, the [Fe xi] 7892Å line, the [Fe xiii] 10747Å and 10798Å lines, and the [Fe xiv] 5303Å line and on some days made only in the [Fe xi] 7892Å and [Fe x] 6374Å lines. Using temperature sensitive emission line ratios [Fe xiv] 5303Å/[Fe xiii] 10747Å and [Fe xi] 7892Å/[Fe x] 6374 Å, we compute the electron temperatures along 18 different loop structures observed on different days. We find a significant negative temperature gradient in all of the structures observed in Fe xiv and Fe xiii and a positive temperature gradient in the structures observed in Fe xi and Fe x. Combining these results with the previous investigations by Singh and his collaborators, we infer that the loop tops, in general, appear hotter when observed in colder lines and colder when observed in relatively hotter lines as compared to their coronal foot points. We suggest that this contrasting trend observed in the temperature variation along the loop structures can be explained by a gradual interaction of different temperature plasma. The exact mechanism responsible for this interaction must be investigated further and has the potential to constrain loop heating models.