Thermal energy budget of electrons in the inner heliosphere: Parker Solar Probe Observations

TL;DR

This study analyzes Parker Solar Probe data to understand the electron thermal energy budget in the inner heliosphere, revealing the necessity of an irreversible heating process likely driven by turbulence.

Contribution

It provides the first detailed observational assessment of the electron thermal energy budget, separating reversible and irreversible processes in the inner heliosphere.

Findings

Irreversible thermal energy source is present from 0.15 to 0.47 au.

Heat flux divergence is positive below 0.33 au and degrades beyond.

Expansion effects dominate below 0.3 au, and turbulence likely drives heating.

Abstract

We present an observational analysis of the electron thermal energy budget using data from Parker Solar Probe. We use the macroscopic moments, obtained from our fits to the measured electron distribution function, to evaluate the thermal energy budget based on the second moment of the Boltzmann equation. We separate contributions to the overall budget from reversible and irreversible processes. We find that an irreversible thermal energy source must be present in the inner heliosphere over the heliocentric distance range from 0.15 to 0.47 au. The divergence of the heat flux is positive at heliocentric distances below 0.33 au, while beyond 0.33 au, there is a measurable degradation of the heat flux. Expansion effects dominate the thermal energy budget below 0.3 au. Under our steady-state assumption, the free streaming of the electrons is not sufficient to explain the observed thermal energy density budget. We conjecture that the most likely driver for the required heating process is turbulence. Our results are consistent with the known nonadiabatic polytropic index of the electrons, which we measure as 1.18 in the explored range of heliocentric distances.