Thermal and Turbulence Characteristics of Fast and Slow Coronal Mass Ejections at 1 AU

Abstract

Understanding the thermal and turbulence properties of interplanetary coronal mass ejections (ICMEs) is essential for analyzing their evolution and interactions with the surrounding medium. This study explores these characteristics across different regions of two distinct ICMEs observed at 1 AU, utilizing in-situ measurements from the Wind spacecraft. The polytropic indices, Gamma_e for electrons and Gamma_p for protons) reveal significant deviations from adiabatic expansion, suggesting sustained heating mechanisms within the ICMEs even at 1AU. The effective polytropic index (Gamma_eff) of the magnetic ejecta (ME) in both ICME1 and ICME2 is found to be near-isothermal (Gamma_eff = 0.88 and 0.76), aligning with measurements near the Sun, highlighting consistent heating across heliospheric distances. Spectral analysis at the inertial scale reveals Kolmogorov-like turbulence in the fast ICME1's ME, while the ME of the slower ICME2 exhibits less developed turbulence with a shallower spectral index (alpha_B). The turbulence analysis in the dissipation scale indicates that the ME of slower ICME2 is less affected by the ambient medium than the faster ICME2. The MEs of both ICMEs show magnetic compressibility much smaller than unity (C_B < 1), suggesting dominant Alfvenic fluctuations in the MEs. Notably, the partial variance of increments (PVI) method identifies more intermittent structures, such as current sheets and reconnection sites, in the sheath and post-ICME regions. Higher PVI values correlate with regions of increased electron and proton temperature (for the sheath region), as well as higher C_B values, highlighting their role in local energy dissipation. These results underscore the importance of ongoing heating and turbulence processes in shaping the evolution of ICMEs.