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The thermal structure of a coronal loop, both along and across the loop, isvital in determining the exact plasma heating mechanism. High resolutionspectroscopic observations of the off-limb corona were made using 25 cmNorikura coronagraph, located at Norikura, Japan. Observations on a number ofdays, 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 theelectron temperatures along 18 different loop structures observed on differentdays. We find a significant negative temperature gradient in all the structuresobserved in Fe XIV and Fe XIII and a positive temperature gradient in thestructures observed in Fe XI and Fe X. Combining these results with theprevious investigations by Singh and his collaborators, we infer that the looptops, in general, appear hotter when observed in colder lines and colder whenobserved in relatively hotter lines as compared to their coronal foot points.We suggest that this contrasting trend observed in the temperature variationalong the loop structures can be explained by a gradual interaction ofdifferent temperature plasma. The exact mechanism responsible for thisinteraction needs further quest and has potential to constrain loop heatingmodels.

Thermal structure of coronal loops as seen with Norikura coronagraph