Thermal Energy Budget of Electrons in the Inner Heliosphere: Parker Solar Probe Observations

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 equa...

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Bibliographic Details
Published in:The Astrophysical journal 2022-12, Vol.941 (2), p.145
Main Authors: Abraham, Joel B., Verscharen, Daniel, Wicks, Robert T., Agudelo Rueda, Jeffersson A., Owen, Christopher J., Nicolaou, Georgios, Jeong, Seong-Yeop
Format: Article
Language:English
Subjects:
Astrophysics
Boltzmann equation
Boltzmann transport equation
Distribution functions
Divergence
Electron distribution
Electrons
Energy budget
Energy sources
Heat flux
Heat transfer
Heliosphere
Irreversible processes
Plasma physics
Solar probes
Solar wind
The Sun
Thermal energy
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Summary: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.
ISSN:0004-637X
1538-4357
DOI:10.3847/1538-4357/ac9fd8