Modeling the thermodynamic evolution of Coronal Mass Ejections (CMEs) using their kinematics

TL;DR

This study models the thermodynamic evolution of a CME during its heliospheric propagation using an improved flux rope model constrained by observed kinematics, revealing heat release and internal forces driving expansion.

Contribution

The paper introduces an enhanced flux rope model to analyze CME thermodynamics throughout its journey, integrating observational data for detailed internal state evolution.

Findings

CME polytropic index decreased from 1.8 to 1.35 during propagation

CME released heat initially and then absorbed heat as it expanded

Thermal force drives CME expansion, while Lorentz force opposes it

Abstract

Earlier studies on Coronal Mass Ejections (CMEs), using remote sensing and in situ observations, have attempted to determine some of the internal properties of CMEs, which were limited to a certain position or a certain time. For understanding the evolution of the internal thermodynamic state of CMEs during their heliospheric propagation, we improve the self-similar flux rope internal state (FRIS) model, which is constrained by measured propagation and expansion speed profiles of a CME. We implement the model to a CME erupted on 2008 December 12 and probe the internal state of the CME. It is found that the polytropic index of the CME plasma decreased continuously from 1.8 to 1.35 as the CME moved away from the Sun, implying that the CME released heat before it reached adiabatic state and then absorbed heat. We further estimate the entropy changing and heating rate of the CME. We also find that the thermal force inside the CME is the internal driver of CME expansion while Lorentz force prevented the CME from expanding. It is noted that centrifugal force due to poloidal motion decreased with the fastest rate and Lorentz force decreased slightly faster than thermal pressure force as CME moved away from the Sun. We also discuss the limitations of the model and approximations made in the study.