Particle-in-cell simulations of Alfv\'en wave parametric decay in a low-beta plasma

TL;DR

This study uses kinetic simulations to analyze how Alfvén waves decay in low-beta plasmas, revealing energy transfer mechanisms that preferentially heat protons and also energize electrons, relevant for solar wind models.

Contribution

First kinetic simulation study of Alfvén wave parametric decay focusing on energy transfer and particle heating in low-beta plasma.

Findings

Protons gain about 50% of the pump wave energy as internal energy.

Electrons gain approximately 10% of the pump wave energy.

Compressible fluctuations and nonlinear effects drive particle heating.

Abstract

We study the parametric decay instability of parallel propagating Alfv\'en wave in a low-beta plasma using one-dimensional fully kinetic simulations. We focus for the first time on the conversion of the energy stored in the initial Alfv\'en wave into particle internal energy, and on its partition between particle species. We show that compressible fluctuations generated by the decay of the pump wave into a secondary ion-acoustic mode and a reflected Alfv\'en wave contribute to the gain of internal energy via two distinct mechanisms. First, the ion-acoustic mode leads nonlinearly to proton trapping and proton phase space mixing, in agreement with previous work based on hybrid simulations. Second, during the nonlinear stage, a compressible front of the fast type develops at the steepened edge of the backward Alfv\'en wave leading to a field-aligned proton beam propagating backwards at the Alfv\'en speed. We find that parametric decay heats preferentially protons, which gain about 50% of the pump wave energy in the form of internal energy. However, we find that electrons are also energized and that they contribute to the total energy balance by gaining 10% of the pump wave energy. By investigating energy partition and particle heating during parametric decay, our results contribute to determine the role of compressible and kinetic effects in wave-driven models of the solar wind.