Ion Energization and Thermalization in Magnetic Reconnection Exhaust Region in the Solar Wind

Plasma energization and thermalization in magnetic reconnection is an important topic in astrophysical studies. We select two magnetic reconnection exhausts encountered by Solar Orbiter and analyze the associated ion heating in the kinetic regime. Both cases feature asymmetric plasma merging in the exhaust and anisotropic heating. For a quantitative investigation of the associated complex velocity-space structures, we adopt a three-dimensional Hermite representation of the proton velocity distribution function to produce the distribution of Hermite moments. We also derive the enstrophy and Hermite spectra to analyze the free energy conversion and transfer in phase space. We find a depletion of Hermite power at small m (corresponding to large-scale structures in velocity space) inside the reconnection exhaust region, concurrent with enhanced proton temperature and decreased enstrophy. Furthermore, the slopes of the 1D time-averaged parallel Hermite spectra are lower inside the exhaust and consistent with the effect of phase mixing that creates small fluctuations in velocity space. These fluctuations store free energy at higher m and are smoothed by weak collisionality, leading to irreversible thermalization. We also suggest that the perpendicular heating may happen via perpendicular phase mixing resulting from finite Larmor radius effects around the exhaust boundary.

Automated summary

This study focuses on ion heating mechanisms in two magnetic reconnection exhausts encountered by Solar Orbiter. Using a three-dimensional Hermite representation, the complex velocity-space structures are quantitatively investigated. Analysis of the enstrophy and Hermite spectra reveals energy conversion and transfer in phase space. The findings indicate a depletion of Hermite power at small m inside the reconnection exhaust region, accompanied by increased proton temperature and decreased enstrophy. The slopes of the 1D parallel Hermite spectra suggest the effect of phase mixing and weak collisionality in the thermalization process. The perpendicular heating is proposed to occur through perpendicular phase mixing due to finite Larmor radius effects.