Effective polytropic index in stream interaction regions near L1 and the effect of temperature anisotropy and plasma β

¹ Department of Physics, National and Kapodistrian University of Athens, Athens, Greece
² Mullard Space Science Laboratory, University College London, Dorking, Surrey RH5 6NT, UK
³ Princeton University, Princeton, NJ 08544, USA

^⋆ Corresponding author: ckatsavrias@phys.uoa.gr

Received: 25 June 2025
Accepted: 5 August 2025

Abstract

Context. Stream interaction regions (SIRs) are large-scale solar wind structures that play a significant role in space weather in the near-Earth environment. Understanding their thermodynamic behaviour is essential to explaining the energy transfer processes governing their evolution.

Aims. We investigate in depth (for the first time to such an extent) the behaviour of both the total and partial proton polytropic indices in SIRs and the high-speed streams (HSSs).

Methods. To that end, we used a list of 186 SIRs identified from Wind measurements over more than two solar cycles (1995–2022), from which we derived the distributions of the polytropic index in the near-Earth space (L1).

Results. Our results show that the slow compressed solar wind region has a sub-adiabatic polytropic index, which indicates compression and turbulent heating. The HSS exhibits a super-adiabatic polytropic behaviour, which is consistent with a decrease in the effective degrees of freedom and/or an additional energy release mechanism. We discuss the consistency of our findings with the fluctuating-moment effect in large-scale compressive fluctuations as such an energy release mechanism.