| Summary, etc. |
Coronal Mass Ejections (CMEs) are powerful solar eruptions that expel magnetized plasma into the heliosphere, often triggering severe space weather disturbances at Earth. While their large-scale kinematics and magnetic structures are well studied, the internal thermodynamic evolution of CMEs, from the low corona to 1 au, remains inadequately understood. This thesis aims to bridge this gap through a comprehensive investigation that integrates analytical modeling, remote observations, in situ measurements, and statistical analysis. One of the core focuses of the thesis is the development and refinement of the Flux Rope Internal State (FRIS) model, an analytical framework that derives internal thermodynamic parameters, such as polytropic index, heating rate, and internal forces, using the remotely measurable kinematic quantities from coronagraphs (SOHO/LASCO and STEREO/COR). The original formulation of FRIS is critically re-evaluated, and key mathematical inconsistencies are corrected to ensure physical self-consistency in the derived quantities. This improved model is then applied to selected CMEs with contrasting kinematics, revealing detailed insights into their thermal state evolution and internal force balance during the early propagation phase. These results are further compared with in situ analyses of two selected ICMEs, where sustained heating is indicated by near-isothermal effective polytropic indices (Γ ≈ 0.88 & 0.76) and supported by turbulence diagnostics, such as inertial and dissipation-scale spectral slopes, low magnetic compressibility, and enhanced intermittency in the sheath and post- ICME regions, reinforcing the continuity of heating from the low corona to 1au. Beyond case-specific implementation, a focused investigation of nine Earth-directed, fast CMEs using the improved FRIS model and 3D kinematics derived from the Graduated Cylindrical Shell (GCS) reconstruction reveals a two-phase thermal evolution: an initial heat-release phase (Γ > 5/3) followed by a transition into a nearly isothermal heating state (Γ ≈ 0.8–1.2) at heliocentric distances between 3–7 𝑅⊙. This evolution is closely coupled with the CMEs’ expansion behavior, where CMEs with slower expansion acceleration show less pronounced temperature drops before reaching isothermal conditions. A detailed internal force balance analysis indicates that Lorentz forces act to suppress expansion during the early propagation phase, whereas thermal pressure and centrifugal forces drive the expansion, with thermal pressure becoming the dominant contributor at larger heliocentric distances. Differential emission measure (DEM) analysis of the CME source regions using SDO/AIA EUV data corroborates the presence of intrinsically hot plasma (2.8–7.2 MK), in agreement with the FRIS-derived These findings challenge the often assumed constancy of Γ in CME modeling and highlight the necessity of incorporating its dynamic evolution into global MHD simulations and predictive tools. The statistical component of the thesis analyzes CMEs across Solar Cycles 23, 24, and the ascending phase of Cycle 25 using in situ observations at 1 au from missions such as Wind and ACE. Within a polytropic framework, the thermal state of magnetic ejecta (MEs) is characterized using event-wise median proton polytropicindices (Γ𝑝), allowing classification into heating and cooling categories. MEs predominantly exhibit non-equilibrium thermal behavior, with a substantial fraction (∼45%) undergoing net heating during interplanetary propagation, especially near solar maxima. In contrast, cooling MEs maintain nearly constant and elevated Γ𝑝 values (∼2), indicating enhanced energy loss beyond adiabatic expectations or possible thermal retention from their eruption. A distinct solar cycle modulation is observed in Γ𝑝, proton temperature, and expansion speed, with the median Γ𝑝 increasing from 1.46 in Solar Cycle 23 to 1.87 in Cycle 24, suggesting a shift toward more cooling-dominated states over time. Despite thermodynamic similarities between magnetic clouds (MCs) and non-cloud ejecta, MCs consistently emerge as the most geoeffective structures, characterized by enhanced magnetic fields, low plasma beta, and suppressed Γ𝑝 values. Further, superposed epoch analyses (SEA) are performed on categorized ICME events, which reveal systematic variations in thermal state, plasma temperature, and magnetic field strength across distinct ICME substructures, including the pre-ICME, sheath, magnetic ejecta, and post-ICME regions. Notably, High-impact ICMEs are predominantly found to be thermally heated, with lower Γ𝑝 values,stronger sheath compression, elevated bulk speeds, and trailing high-speed solar wind, all contributing to enhanced geomagnetic storm potential. Finally, a detailed study of CME–CME interactions associated with the 10 May 2024 great storm shows that such interactions significantly modify both kinematics and thermodynamics of the involved structures. The resulting complex ejecta at 1 au exhibit enhanced magnetic fields, heat-release-dominated electrons,and bimodal ion thermal states. FRIS model analysis reveals diverse thermal behaviors among the individual CMEs, with most exhibiting a transition to an isothermal state by 6–9 𝑅⊙, except in cases of suppressed expansion. The findings underscore that CME–CME interactions can obscure one-to-one comparisons between solar-origin properties and in situ measurements, while also enhancing internal heating, structural complexity, and geo effectiveness. This thesis concludes by synthesizing results from analytical modeling, selected case studies, and long-term statistical analyses to construct a unified understanding of CME thermodynamics in the low corona and at 1 au. The findings reveal that CME thermal evolution is highly variable and depends on factors such as expansion dynamics, Solar Cycle phase, and interplanetary interactions. By integrating remote-sensing and in situ observations within a physically consistent thermodynamic framework, this work successfully bridges the gap between observational regimes, revealing the dynamic interplay between CME kinematics and internal thermal evolution. These insights not only advance the fundamental understanding of CME physics but also offer new pathways for improving space weather forecasting through diagnostics based on CME thermal state. |
