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

Joel B. Abraham, Daniel Verscharen, Robert T. Wicks, Jefferson A. Agudelo Rueda, Christopher J. Owen, Georgios Nicolaou, Seong-Yeop Jeong

Submitted on 3 November 2022


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 a 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 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 non-adiabatic polytropic index of the electrons, which we measure as 1.176 in the explored range of heliocentric distances.


Comment: Paper accepted to The Astrophysical Journal

Subjects: Astrophysics - Solar and Stellar Astrophysics; Physics - Geophysics; Physics - Plasma Physics; Physics - Space Physics