Measurement of the polytropic index during solar coronal rain using a diagram of the electron density distribution as a function of the electron temperature

TL;DR

This study measures the polytropic index of solar coronal plasma during rain events using DEM analysis, revealing a transition from thermally unstable to stable states, which enhances understanding of coronal plasma thermodynamics.

Contribution

It introduces a method to estimate the polytropic index during coronal rain using DEM data, highlighting the thermodynamic transition in the solar corona.

Findings

Polytropic index before rain: 2.1, indicating instability.

Polytropic index after rain: 1.3, indicating stability.

Coronal rain results from thermal instability, with plasma returning to a stable state.

Abstract

A differential emission measure (DEM) method is used to evaluate the relationship of the electron density and temperature before and after a coronal rain event during an active sun over the period from 20:10 UT on October 6 to 02:10 on October 7, 2011. Observational data were obtained from SDO/AIA for six different extreme ultraviolet (EUV) spectral lines. 240 different coronal loops were analyzed during this time interval, and the average electron density and temperature were obtained using 171 {\AA} (Fe IX) and 193 {\AA} (Fe XII) filters. The relationship between the density and temperature made it possible to estimate the polytropic index in the solar corona before and after the coronal rain. The polytropic index after the termination of the coronal rain was estimated to be {\gamma} = 1.3{\pm}0.06, Which shows the usual thermodynamic properties of study-state coronal plasma. The polytropic index at the time of onset of the coronal rain was, however, estimated to be {\gamma} = 2.1{\pm}0.11, which indicates an unstable thermodynamic process, i.e., thermal instability. It is suggested that the coronal rain is the result of an unstable process, and the coronal plasma returns its stable state after the rain.