Radial profile of the polytropic index of solar wind plasma in the heliosphere

TL;DR

This paper synthesizes various measurements of the polytropic index of solar wind plasma across the heliosphere, revealing a decreasing trend with distance and discussing implications for plasma collisionality and wave-particle interactions.

Contribution

It introduces a unified theoretical model of the polytropic index across the heliosphere, integrating diverse observational data and explaining the radial variation.

Findings

Polytropic index decreases with heliocentric distance

Inner heliosheath plasma may be collisionless

Wave energy transfer diminishes with distance

Abstract

We combine different measurements of the polytropic index of the proton plasma in the heliosphere: i) near-adiabatic index in the inner heliosphere ~1AU, ii) subadiabatic indices in the outer heliosphere ~20-40AU, and iii) near-zero indices in the inner heliosheath. These observations are unified by a single theoretical model of the polytropic index throughout its radial extent in the heliosphere; the corresponding fitting reveals the decreasing trend of the polytropic index with increasing heliocentric distance R. We anticipate that with increasing R, (i) the Debye length and mean-free-path decreases; (ii) the Landau damping is less effective, transferring thus less wave energy to particles; and (iii) the collisionality degree increases, indicating that the proton plasma in the inner heliosheath might be collisionless.