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Interplanetary Coronal Mass Ejections (ICMEs) and High Speed Streams (HSSs)are noteworthy drivers of disturbance of interplanetary space. Interactionbetween them can cause several phenomena, such as; generation of waves,enhanced geo-effectiveness, particle acceleration, etc. However, how doesthermodynamic properties vary during the ICME-HSS interaction remain an openproblem. In this study, we investigated the polytropic behavior of plasmaduring an ICME-HSS interaction observed by STEREO and Wind spacecraft. We findthat the ICME observed by the STEREO-A has polytropic index $\alpha = 1.0$,i.e., exhibiting isothermal process. Moreover, Wind spacecraft observed the HSSregion, non-interacting ICME, and ICME-HSS interaction region. During eachregions we found $\alpha$=1.8, $\alpha$=0.7, and $\alpha$=2.5, respectively. Itimplies that the HSS region exhibits a nearly adiabatic behaviour, ICME regionis closely isothermal, and the ICME-HSS interaction region exhibitssuper-adiabatic behaviour. The insufficient expansion of the ICME due to theinteraction with HSS triggers the system for heating and cooling mechanismswhich dependent on the degrees of freedom of plasma components.

Distinct polytropic behavior of plasma during ICME-HSS Interaction