Polytropic Behavior of Solar Wind Protons Observed by Parker Solar Probe

TL;DR

This study analyzes Parker Solar Probe data to understand the polytropic behavior of solar wind protons, revealing different polytropic indices for large-scale expansion and short-time turbulence-related fluctuations.

Contribution

It provides the first detailed analysis of the polytropic index variations of solar wind protons at different scales using in-situ measurements.

Findings

Large-scale proton density and temperature follow a polytropic index ~5/3.

Short-time fluctuations exhibit a higher polytropic index, indicating different physical processes.

Correlations found between the polytropic index and plasma parameters such as speed and magnetic field.

Abstract

A polytropic process describes the transition of a fluid from one state to another through a specific relationship between the fluid density and temperature. The value of the polytropic index that governs this relationship determines the heat transfer and the effective degrees of freedom during the process. In this study, we analyze solar wind proton plasma measurements, obtained by the Faraday cup instrument on-board Parker Solar Probe. We examine the large-scale variations of the proton plasma density and temperature within the inner heliosphere explored by the spacecraft. We also address a polytropic behavior in the density and temperature fluctuations in short-time intervals, which we analyze in order to derive the effective polytropic index of small time-scale processes. The large-scale variations of the solar wind proton density and temperature which are associated with the plasma expansion through the heliosphere, follow a polytropic model with a polytropic index ~5/3. On the other hand, the short time-scale fluctuations which may be associated with turbulence, follow a model with a larger polytropic index. We investigate possible correlations between the polytropic index of short time-scale fluctuations and the plasma speed, plasma beta, and the magnetic field direction. We discuss the scenario of mechanisms including energy transfer or mechanisms that restrict the particle effective degrees of freedom.