Modeling the Thermodynamic Evolution of Coronal Mass Ejections Using Their Kinematics

Abstract

Earlier studies on coronal mass ejections (CMEs), using remote sensing and in situ observations, have attempted to determine some internal properties of CMEs, which were limited to a certain position or a certain time. To understand the evolution of the internal thermodynamic state of CMEs during their heliospheric propagation, we improve the self-similar flux-rope internal state model, which is constrained by the measured propagation and expansion speed profiles of a CME. We implement the model in a CME that 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 an 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 the Lorentz force prevented the CME from expanding. It is noted that the centrifugal force due to poloidal motion decreased at the fastest rate, and the Lorentz force decreased slightly faster than the thermal pressure force as the CME moved away from the Sun. We also discuss the limitations of the model and approximations made in the study.