Studying the polytropic behavior of an ICME using Multi-spacecraft observation by STEREO-A, STEREO-B, and WIND.

The polytropic behavior describes the relationship between macroscopic properties of plasma species and provides useful information in understanding physical mechanisms in space plasma. Through the value of the polytropic index, which governs the polytropic relationship, we can identify the energy transfer in plasma systems. In this paper, we determine the polytropic index of proton plasma within an Interplanetary Coronal Mass Ejection (ICME), which has been observed by three different spacecraft at ∼ 1 AU, each separated by ∼ 20 ∘ degrees in longitude during the analyzed observations. By performing this multi-spacecraft analysis, we provide evidence that the polytropic index does not vary significantly across the structure of the observed ICME. This may imply that the ICME evolution process is nearly coherent. Moreover, the polytropic index values we derive here can set boundary conditions in existing and future ICME evolution models. [ABSTRACT FROM AUTHOR]