Modification of Proton Velocity Distributions by Alfvenic Turbulence in the Solar Wind

TL;DR

This study models how Alfvenic turbulence influences proton velocity distributions in the solar wind, explaining the formation of nonthermal tails observed in situ beyond 0.3 AU.

Contribution

It introduces a numerical kinetic model incorporating Alfven wave turbulence effects to explain proton VDF modifications in the solar wind.

Findings

Turbulence causes rapid development of anti-sunward tails in proton VDFs.

Results explain observed nonthermal tails in solar wind proton distributions.

Model aligns with in-situ observations beyond 0.3 AU.

Abstract

In the present paper, the proton velocity distribution function (VDF)in the solar wind is determined by solving numerically the kinetic evolution equation. We compare the results obtained when considering the effects of ex- ternal forces and Coulomb collisions with those obtained by adding effects of Alfven wave turbulence. We use Fokker-Planck diffusion terms due to Alfvenic turbulence, which take into account observed turbulence spectra and kinetic effects of finite proton gyroradius. Assuming a displaced Maxwellian for the proton VDF at the simulation boundary at 14 solar radii, we show that the turbulence leads to a fast (within several solar radii) development of the anti-sunward tail in the proton VDF. Our results provide a natural explanation for the nonthermal tails in the proton VDFs, which are often observed in-situ in the solar wind beyond 0.3 AU.