Thermal structure of coronal loops as seen with Norikura coronagraph

TL;DR

This study uses high-resolution spectroscopic data from the Norikura coronagraph to analyze the thermal structure of coronal loops, revealing temperature gradients that inform models of plasma heating mechanisms.

Contribution

It provides detailed measurements of temperature gradients along coronal loops using multiple forbidden iron emission lines, enhancing understanding of plasma heating processes.

Findings

Negative temperature gradient in Fe XIV and Fe XIII lines

Positive temperature gradient in Fe XI and Fe X lines

Loop tops appear hotter in colder lines and colder in hotter lines

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 25 cm Norikura coronagraph, located at Norikura, Japan. Observations on a number of days, were made simultaneously in four forbidden iron emission lines namely, [Fe XI] 7892 \AA\ line, [Fe XIII] 10747 \AA & 10798 \AA\ lines and [Fe XIV] 5303 \AA\ line and on some days made only in [Fe XI] 7892 \AA\ and [Fe X] 6374 \AA\ lines. Using the temperature sensitive emission line ratios [Fe XIV] 5303 \AA/[Fe XIII] 10747 \AA, and [Fe XI] 7892 \AA/[Fe X] 6374 \AA\ we compute the electron temperatures along 18 different loop structures observed on different days. We find a significant negative temperature gradient in all 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 needs further quest and has potential to constrain loop heating models.