| Abstract: Coronal Mass Ejections (CMEs) are primary drivers of space weather.
While their kinematic evolution has been extensively studied, the
internal thermodynamic evolution remains limited. Here, we present a
comprehensive multi-event analysis using analytical modeling and
observations, including CME-CME interaction and a statistical analysis
spanning Solar Cycles 23-25, to investigate how CME thermal properties
evolve from the Sun into interplanetary space. Using 3D kinematics as
input to the FRIS model, we derive the evolution of the polytropic
index (Γ), heating and cooling rates, and internal force balances
over 2-20 Rsun, where direct in situ plasma measurements are limited.
We find that fast CMEs exhibit three distinct thermodynamic phases: an
initial heat-release phase, a sustained heating phase, and a
transition toward a near-isothermal regime, typically occurring
between ~3 and 9 Rsun. These results challenge the commonly assumed
constant Γ in CME models and demonstrate that its dynamic evolution
is essential for accurately describing CME propagation. A detailed
case study of the May 2024 great geomagnetic storm, driven by the
interaction of six successive CMEs, reveals a heat-release state in
electrons, a bimodal proton thermal distribution, and localized
heating regions, indicating that CME-CME interactions strongly modify
internal thermodynamic properties of merged ejecta at 1 AU. Notably,
electron thermal states, in particular, emerge as sensitive tracers of
recent interaction history. Statistically, CMEs across Solar Cycles
most exhibit pronounced non-equilibrium thermal behavior, with about
45% remaining in heating states at 1 AU. A solar-cycle dependence is
observed, with a shift toward cooling-dominated states during weaker
cycles. Interestingly, High-impact ICMEs with enhanced geomagnetic
responses show low Γ along with strong magnetic fields, compressed
sheaths, rapid expansion, and trailing high-speed streams. These
findings highlight the importance of incorporating dynamically
evolving thermodynamic states, internal force balance into CME and
space weather modelling frameworks. |
