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

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 Alfvénic 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 Alfvénic 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.

9 pages, 4 figures

References in corpus (10)

• Electrons in the Young Solar Wind: First Results from the Parker Solar Probe
• Hybrid-Kinetic Simulations of Ion Heating in Alfvénic Turbulence
• The Radial Evolution of the Solar Wind as Organized by Electron Distribution Parameters
• Precision Electron Measurements in the Solar Wind at 1 au from NASA's Wind Spacecraft
• On the role of solar wind expansion as a source of whistler waves: scattering of suprathermal electrons and heat flux regulation in the inner heliosphere
• Adaptive critical balance and firehose instability in an expanding, turbulent, collisionless plasma
• Quantifying the Energy Budget in the Solar Wind from 13.3-100 Solar Radii
• Collisionless heat flux regulation via electron firehose instability in presence of a core and suprathermal population in the expanding solar wind
• Proton fire hose instabilities in the expanding solar wind
• A Kinetic Model of Solar Wind Acceleration Driven by Ambipolar Electric Potential and Velocity-Space Diffusion