Dependence of the polytropic behaviour of solar wind protons on temperature anisotropy and plasma β near L1

Abstract

Context. A polytropic process describes the transition of a fluid from one state to another through a specific relationship between the fluid density and temperature, while the value of the polytropic index that governs this relationship determines the heat transfer and the effective degrees of freedom of that specific process.Aims. In this paper we investigate in depth the relationship between the proton effective polytropic index γ in the solar wind, the proton anisotropy α, and plasma β, while – for the first time to our knowledge to such an extent – we further investigate the dependence of the partial (with respect to the magnetic field) polytropic index to both the above-mentioned plasma parameters.Methods. To this end we use the entire Wind dataset spanning the 1995 to 2023 time period to derive the distributions of the polytropic index in the near-Earth space (L1).Results. Our results indicate that the proton γ increases with increasing proton anisotropy and decreases with increasing plasma β. Finally, we show that even though the average (over long time periods) total and partial proton polytropic index values are very close, these values correspond to isotropic plasma alone, with a further balance between the thermal and magnetic pressure.On the contrary, for shorter time periods and/or specific solar wind structures, where the proton anisotropy and plasma β exhibit deviations from these average values, the partial proton polytropic index exhibits significant variation that is dependent on the anisotropy and on plasma β.