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

TL;DR

This study addresses how the thermal evolution of interplanetary magnetic ejecta (ME) within ICMEs at 1 AU relates to their geoeffectiveness across Solar Cycles 23–25. It employs a polytropic framework, extracting the proton polytropic index $\Gamma_p$ from 1-minute in-situ OMNI measurements and classifying MEs into Heating and Cooling populations, with Superposed Epoch Analysis (SEA) used to derive median profiles across pre-ICME, sheath, ME, and post-ICME regions. Key findings reveal solar-cycle modulation of ME thermodynamics, with Heating MEs dominating near solar maxima and descending phases, SC23 displaying stronger heating signatures than SC24, and High-impact ICMEs often being Heating MEs with stronger magnetic fields and deeper geomagnetic storms. The results demonstrate that $\Gamma_p$ complements magnetic and dynamic properties as a diagnostic of ICME energetics and geoeffectiveness, offering a pathway to improved space-weather forecasting by incorporating thermal evolution into CME characterization.

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%) 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 = 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.

Figures (9)

• Figure 1: Annual occurrences of (a) ICMEs and (b) MC and non-MC events across Solar Cycles 23, 24, and the rising phase of 25. Yearly mean sunspot numbers are overplotted in both panels for comparison with solar activity levels.
• Figure 2: Derived $\Gamma_p$ values across the ICME event on 28 May 2010. The gray dashed vertical line marks the start of the sheath region, while the black dashed vertical lines denote the boundaries of ME. Orange dots represent reliable $\Gamma_p$ values, whereas gray dots indicate less reliable estimates. The error bars represent the uncertainty due to measurement uncertainty in $N_p$ and $T_p$. The green dashed horizontal line marks the adiabatic index ($\Gamma_p = 5/3$) for reference. The right panel shows the histogram of reliable $\Gamma_p$ values within ME.
• Figure 3: (a) Annual occurrence of heating and cooling ME across SC23, 24 and rising phase of SC25. (b) Annual distribution of ME with various categories depending on $\Gamma_p$ values.
• Figure 4: Distribution of $\Gamma_p$ over in SC23, 24 and ascending phase of SC25
• Figure 5: Yearly median values of (a) polytropic index ($\Gamma_p$), (b) proton temperature ($T_p$), (c) proton number density ($N_p$), (d) magnetic field strength ($B$), (e) plasma beta ($\beta$), (f) bulk speed ($V$), and (g) expansion speed ($V_{exp}$) for heating and Cooling MEs across Solar Cycles 23, 24, and the ascending phase of Cycle 25. The background color-shaded regions represent the 68% (1$\sigma$) confidence intervals obtained via bootstrap resampling. Each panel is overplotted with the yearly mean sunspot numbers to investigate the correlation between solar activity and the thermal, plasma, and magnetic properties of MEs.