Thermal properties of interplanetary coronal mass ejections at 1 au and their connection to geoeffectiveness across solar cycles 23–25

ABSTRACT Interplanetary coronal mass ejections (ICMEs) are major drivers of heliospheric variability and can produce prolonged disturbances near Earth. Understanding their thermodynamic evolution is crucial for assessing their heat budget and exploring how thermal states relate to their plasma dynamics and geoeffectiveness. We conduct a comprehensive statistical analysis of magnetic ejecta (MEs) over solar cycles 23, 24, and the ascending phase of 25. Leveraging a polytropic framework, we characterized the thermal state of ME based on the event-wise median proton polytropic index ($\Gamma _p$) from in-situ measurements at 1 au. We find that MEs are thermodynamically active and rarely evolve adiabatically or isothermally. Notably, a significant fraction (45 per cent) of MEs exhibit a heating state. Heating MEs dominate near solar maxima and exhibit strong solar-cycle modulation in $\Gamma _p$, proton temperature, and expansion speed, indicating active in-transit heating processes. Whereas, cooling MEs show a nearly constant $\Gamma _p \sim 2$ across cycles, suggesting enhanced cooling beyond adiabatic expectations and possible thermal energy retention from eruption to 1 au. Notably, the median $\Gamma _p$ value increases from 1.49 (SC23) to 1.88 (SC24), indicating a shift to cooling-dominated states over successive cycles. High-impact ICMEs, predominantly heating MEs ($\Gamma _p = 0.59$), often manifest as magnetic clouds with enhanced magnetic fields, low plasma beta, pronounced sheath compression, elevated expansion, and post-ICME high-speed flows, making them the most geoeffective drivers of strong geomagnetic storms. These results establish $\Gamma _p$ as a useful diagnostic of ICME thermal states, though meaningful assessment of geoeffectiveness requires combined consideration of thermal, plasma, and magnetic field properties.