Thermodynamic Characterization of Solar Wind Interaction Regions Using PSP Observations: Evidence of Subadiabatic and Superadiabatic States

This study provides a detailed analysis of the thermodynamic behavior within stream interaction regions (SIRs), focusing on an effective polytropic index assessment of 25 distinct events. To improve the accuracy of the effective polytropic index estimation, we applied a multistep filtering method based on the Bernoulli integral to remove outliers caused by streamline mixing and correlation coefficients to improve the applicability of the polytropic model. Considering the effective degree of freedom ( f = 3), 6 out of the 25 analyzed events show superadiabatic behavior, and the remaining events span subadiabatic, adiabatic, and superadiabatic states. The distribution of the determined polytropic indices for all the events reveals two distinct thermodynamic regimes, with peaks at γ = 1.22 (subadiabatic) and γ = 2.34 (superadiabatic). This analysis offers valuable insights into the complex thermodynamic processes occurring within solar wind interactions. The effective polytropic index of the solar wind does not have a single characteristic value throughout the heliosphere. Instead, it reflects varying heating and cooling processes and/or degrees of freedom within these structures, contributing to a deeper understanding of SIR dynamics.