The Polytropic Index of Interplanetary Coronal Mass Ejections near L1

Abstract

Context. A polytropic process describes the transition of a fluid from one state to another through a specific relationship between the fluid density and temperature, and the value of the polytropic index that governs this relationship determines the heat transfer and the effective degrees of freedom of this specific process.Aims. In this paper, we investigate in depth the proton polytropic behaviour in interplanetary coronal mass ejections (ICMEs). Moreover, for the first time (to our knowledge and at such an extent) we further investigate the behaviour of both the total and partial polytropic indices within ICMEs with various magnetic field configurations inside the magnetic obstacles.Methods. To that end we used a list of 401 ICMEs identified from Wind measurements during more than two solar cycles (1995–2001), during which we derived the distributions of the polytropic index in the near-Earth space (L1).Results. Our results show that sheaths are sub-adiabatic, indicating turbulent plasma, while the value of γ further depends on the existence of a shock. Furthermore, the polytropic behaviour of the protons inside the ICME magnetic obstacles is dependent on the magnetic field configuration, with flux ropes with rotation above 90 deg exhibiting sub-adiabatic γ, while ejecta with no clear rotation exhibiting super-adiabatic γ, supporting the scenario that changes during the interplanetary evolution might affect the magnetic field configuration inside the magnetic obstacle.