Understanding the Thermal Properties of Fast CMEs by Integrating White-light Observations and Analytical Modeling

Abstract

Coronal Mass Ejections (CMEs), huge magnetized plasma erupting from the Sun, pose potential risks to space weather and space-based infrastructure. While extensive research has focused on examining the kinematics of CMEs, there has been limited study of their thermodynamic evolution, particularly at specific heliocentric distances closer to the Sun. Acknowledging that variations in internal plasma properties can impact the overall evolution of CMEs and vice versa is crucial. This study investigates diverse kinematic profiles and associated thermodynamic changes in nine fast CMEs at coronal heights where measuring thermodynamics is challenging. We estimated the distance-dependent evolution of various internal parameters, including polytropic index, temperature, heating rate, pressure, and internal forces driving CME expansion by leveraging the improved Flux Rope Internal State (FRIS) model. The FRIS model utilizes the 3D kinematics derived from the Graduated Cylindrical Shell (GCS) model as input. Our findings reveal that CMEs can maintain their temperature above the adiabatic cooling threshold despite expansion, progressing towards an isothermal state during later propagation phases. The fast CMEs maintaining higher expansion speeds exhibit less pronounced temperature decreases. We found that CME's expansion speed and acceleration correlate well with its maximum temperature drop to reach the isothermal state. Multi-wavelength observations of flux ropes at the source region support the FRIS model-derived results at lower coronal heights. Our analysis elucidates that the primary forces influencing CME radial expansion are the centrifugal and thermal pressure forces, while the Lorentz force acts as a constraining factor. Notably, the thermal pressure force governs expansion at higher heights and is solely responsible for radial expansion. This study enhances our comprehension of the thermodynamic properties of fast CMEs, offering valuable insights for refining assumptions of the polytropic index value in different magnetohydrodynamics (MHD) models to improve the prediction of CME properties at 1 AU.