Formation of Suprathermal Electron Populations in the Expanding, Turbulent Solar Wind

TL;DR

This study uses advanced kinetic simulations to reveal how solar wind expansion and turbulence jointly produce nonthermal, suprathermal electron populations with power-law tails, highlighting the role of parallel energization mechanisms.

Contribution

First fully kinetic simulation demonstrating the combined effects of expansion and turbulence in generating nonthermal electron features in the solar wind.

Findings

Suprathermal electrons develop in the parallel direction with power-law tails.

Expansion-driven cooling pushes plasma toward firehose instability threshold.

Parallel energization likely involves electric fields or wave-particle interactions.

Abstract

Nonthermal features are ubiquitously observed in electron velocity distribution functions in the solar wind, yet their origin in the collisionless, turbulent, expanding solar-wind plasma remains unclear. We investigate how solar-wind expansion and Alfv\'enic turbulence jointly generate and regulate these features using the first fully kinetic particle-in-cell simulation of an expanding turbulent plasma under heliospheric conditions. In our setup, expansion-driven weakening of the magnetic field adiabatically cools the plasma perpendicularly to the mean field while leaving the parallel temperature largely unchanged, driving the system toward the firehose-instability threshold. Concurrently, strongly anisotropic turbulence leads to perpendicular heating and the development of nonthermal features. Subsequently, we find that suprathermal electron populations preferentially develop in the parallel direction, forming pronounced power-law tails even under weakly compressive, highly Alfv\'enic conditions, and persist despite anisotropy regulation by the firehose instability. The preferentially parallel energization suggests the involvement of parallel electric fields or resonant wave--particle interactions, rather than simple velocity-space redistribution. These results provide the first direct evidence of the emergence of nonthermal-electron features in a unified kinetic framework linking expansion, turbulence, and instabilities in the solar wind.