The polytropic index of solar coronal plasma in sunspot fan loops and its temperature dependence

TL;DR

This study investigates the polytropic index of solar coronal plasma in sunspot fan loops using SDO/AIA data, revealing a temperature-dependent increase in the index and suggesting suppressed thermal conduction at higher temperatures.

Contribution

It provides the first comprehensive analysis of the temperature dependence of the polytropic index across multiple active regions in sunspot fan loops.

Findings

Polytropic index varies from 1.04 to 1.58 across different loops.

Higher polytropic indices are observed at hotter temperatures.

Phase shifts between temperature and density oscillations are larger than classical thermal conduction predictions.

Abstract

Observations of slow magneto-acoustic waves have been demonstrated to possess a number of applications in coronal seismology. Determination of the polytropic index ($\gamma$) is one such important application. Analysing the amplitudes of oscillations in temperature and density corresponding to a slow magneto-acoustic wave, the polytropic index in the solar corona has been calculated and based on the obtained value it has been inferred that thermal conduction is highly suppressed in a very hot loop in contrast to an earlier report of high thermal conduction in a relatively colder loop. In this study, using SDO/AIA data, we analysed slow magneto-acoustic waves propagating along sunspot fan loops from 30 different active regions and computed polytropic indices for several loops at multiple spatial positions. The obtained $\gamma$ values vary from 1.04$\pm$0.01 to 1.58$\pm$0.12 and most importantly display a temperature dependence indicating higher $\gamma$ at hotter temperatures. This behaviour brings both the previous studies to agreement and perhaps implies a gradual suppression of thermal conduction with increase in temperature of the loop. The observed phase shifts between temperature and density oscillations, however, are substantially larger than that expected from a classical thermal conduction and appear to be influenced by a line-of-sight integration effect on the emission measure.