Revealing super-adiabatic features of interplanetary coronal mass ejections at 1 au

Abstract

Interplanetary coronal mass ejections (ICMEs) are large-scale, coherent magnetic structures that play a pivotal role in heliospheric dynamics and space weather phenomena. Although thermodynamic analyses of ICME magnetic obstacles (MOs) at 1 au generally reveal adiabatic characteristics, the broader thermodynamic processes and associated plasma heating and cooling mechanisms remain insufficiently understood. In this study we analysed 473 ICME MOs observed at 1 au by the ACE spacecraft, utilising polytropic analysis to determine the polytropic index, α, for these structures. We identified 25 ICME MOs in which plasma protons exhibit a polytropic index α ≳ 2.00, with a mean value of 2.14 ± 0.07, indicating super-adiabatic behaviour. We also observed evidence of 12 isothermal (α ∼ 1) and 45 adiabatic (α ∼ 5/3) ICME MOs. Furthermore, in the case of super-adiabatic ICME MOs, we observe that all the total supplied heat is efficiently utilised to accomplish work in the surrounding environment, assuming the protons have three effective kinetic degrees of freedom. Therefore, as they expand, these ICMEs MOs cool faster than the adiabatic plasma. Our findings are critical to comprehending the dynamic evolution of ICMEs in interplanetary space and the energy-exchange mechanisms involved.