Fluctuating polytropic processes, turbulence, and heating

TL;DR

This paper develops a thermodynamic framework for fluctuating polytropic processes to explain turbulence-induced heating in solar wind plasma, deriving analytical profiles consistent with observations.

Contribution

It introduces a novel thermodynamic description of turbulent heating via fluctuating polytropic processes and applies it to solar wind plasma and pickup ion energy transfer.

Findings

Fluctuating polytropic processes produce net heating even when nonfluctuating processes are adiabatic.

Analytical profiles of turbulent and fluctuating polytropic heating are identical.

The model fits observed PUI heating rates and explains subadiabatic cooling in the solar wind.

Abstract

This paper explores the thermodynamics of fluctuating polytropic processes and their connection to turbulence. It is shown that random fluctuations of polytropic processes produce a nonzero overall heating of a particle system, e.g., solar wind plasma flowing out through the heliosphere; while any nonturbulent heating can be thermodynamically described by typical nonfluctuating polytropic processes, turbulent heating can be thermodynamically described through fluctuating polytropic processes. First, we derive the expression of the overall process and find that polytropic fluctuations lead to heat entering the system even if the respective nonfluctuating process is adiabatic. The temperature of the solar wind plasma protons decreases with heliospheric distance less than the adiabatic cooling, again, similar to when heating enters the system; this subadiabatic cooling is proportional to the variance of the fluctuations. We derive the heliospheric radial profiles of the thermodynamic expressions of the polytropic index, temperature, and heating rates. Then, we show that the analytical profiles of heating of fluctuating polytropic processes and of turbulent heating are identical, suggesting that turbulence heats plasma particle populations by fluctuating their polytropic processes. We apply the thermodynamics of fluctuating polytropic processes to the energy transfer from pickup ions (PUIs) to solar wind plasma protons, and derive the analytical expressions of PUI turbulent and nonturbulent heating rates, which are well fitted to the respective observations. Finally, we apply the thermodynamic model to the radial profile of PUI energy transfer to the solar wind plasma protons, where we derive the portion of PUI turbulent vs. nonturbulent heating rates.