Thermodynamics of Collision-Dominated Expanding Plasma: Heating of ICMEs

A recent statistical study of interplanetary coronal mass ejections (ICMEs) between 0.3 and 5.4 AU (Liu et al. 2005) shows that ICMEs expand during their propagation through the heliosphere: the density drops faster than r-2, and the magnetic field magnitude also exhibits a steeper decrease with distance than the typical solar wind. The expansion, however, does not accelerate the cooling of ICMEs and seems to be governed by a polytrope with γ = 1.1-1.2. The ICME data reveal that the ratio of the expansion time to the Coulomb collision time is usually larger than unity inside ICMEs, so Coulomb collisions are important contributors to the ion-ion equilibration process. As expected for a collision-dominated plasma, the alpha-proton differential speed quickly drops below 10 km s-1. The temperature ratio of alpha particles to protons, in contrast, is even higher within ICMEs than in the ambient solar wind, suggestive of a preferential heating of alpha particles. Taking into account the expansion and energy transfer between protons and alpha particles via Coulomb collisions, we model the thermodynamics of ICMEs. The heating rate as a function of heliocentric distance required for the temperature profile is deduced. We also examine the role of turbulence dissipation in the local heating of ICMEs at 0.3-5.4 AU, using high time-resolution magnetic filed observations. The turbulence cascade rate is thereby inferred from the inertial range power spectrum of magnetic fluctuations, based on Kolmogoroff's law and its MHD equivalent, Kraichnan's formulation, respectively. We will compare the required heating rate with the turbulence dissipation rate. In addition, turbulence generated by micro-instabilities driven by temperature anisotropies will be investigated. Preliminary results show that the ICME plasma is not near the thresholds for instabilities, so these instabilities may not contribute to the ICME heating.