Three dimensional particle-in-cell simulation of particle acceleration by circularly polarised inertial Alfven waves in a transversely inhomogeneous plasma

TL;DR

This study uses 3D particle-in-cell simulations to investigate how circularly polarised inertial Alfven waves accelerate particles and heat ions in a transversely inhomogeneous plasma, relevant to solar and space phenomena.

Contribution

It presents the first fully-kinetic 3D simulation demonstrating electron acceleration and ion heating by IAWs in inhomogeneous plasma, aligning with solar flare observations.

Findings

Electron acceleration efficiency of ~45% in density inhomogeneity regions.

Ion transverse heating of 75%, twice as high as previous 2.5D studies.

Parallel electric fields are localized and rotate with the IAWs.

Abstract

The process of particle acceleration by left-hand, circularly polarised inertial Alfven waves (IAW) in a transversely inhomogeneous plasma is studied using 3D particle-in-cell simulation. A cylindrical tube with, transverse to the background magnetic field, inhomogeneity scale of the order of ion inertial length is considered on which IAWs with frequency $0.3 \omega_{ci}$ are launched that are allowed to develop three wavelength. As a result time-varying parallel electric fields are generated in the density gradient regions which accelerate electrons in the parallel to magnetic field direction. Driven perpendicular electric field of IAWs also heats ions in the transverse direction. Such numerical setup is relevant for solar flaring loops and earth auroral zone. This first, 3D, fully-kinetic simulation demonstrates electron acceleration efficiency in the density inhomogeneity regions, along the magnetic field, of the order of 45% and ion heating, in the transverse to the magnetic field direction, of 75%. The latter is a factor of two times higher than the previous 2.5D analogous study and is in accordance with solar flare particle acceleration observations. We find that the generated parallel electric field is localised in the density inhomogeneity region and rotates in the same direction and with the same angular frequency as the initially launched IAW. Our numerical simulations seem also to suggest that the "knee" often found in the solar flare electron spectra can alternatively be interpreted as the Landau damping (Cerenkov resonance effect) of IAWs due to the wave-particle interactions.