Thermal and turbulence characteristics of fast and slow coronal mass ejections at 1 AU

Abstract

Understanding thermal and turbulence properties of interplanetary coronal mass ejections (ICMEs) is essential for analysing 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. Polytropic indices (e for electrons and p for protons) reveal significant deviations from adiabatic expansion, suggesting sustained heating mechanisms within the ICMEs even at 1 AU. Effective polytropic index (eff) of the magnetic ejecta (ME) in both ICME1 and ICME2 is found to be near-isothermal (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 ME of the slower ICME2 exhibits less-developed turbulence with a shallower spectral index (αB). 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 (CB < 1), suggesting dominant Alfvénic fluctuations in the MEs. Notably, the partial variance of increments (PVI) method identifies more intermittent structures, such as current sheets and reconnection sites, in sheath and post-ICME regions. Higher PVI values correlate with regions of increased electron and proton temperatures (for the sheath region) as well as higher CB values, highlighting their role in local energy dissipation. These results enchance the importance of ongoing heating and turbulence processes in shaping the evolution of ICMEs.