Resonance of low-frequency electromagnetic and ion-sound modes in the solar wind

TL;DR

This paper demonstrates through simulations that low-frequency electromagnetic waves in the solar wind can resonate with ion-sound modes, leading to energy transfer and plasma density steepening, especially for oblique wave propagation.

Contribution

It reveals a nonlinear two-wave resonance process in the solar wind, showing how electromagnetic fluctuations can transfer energy to electrostatic modes via ion-sound resonance.

Findings

Resonance occurs at a specific wavenumber related to sound and Alfvén speeds.

Oblique propagation is necessary for the resonance to occur.

The process can frequently happen in the solar wind, facilitating energy transfer.

Abstract

Parker Solar Probe measurements have recently shown that coherent fast magnetosonic and Alfv\'{e}n ion-cyclotron waves are abundant in the solar wind and can be accompanied by higher-frequency electrostatic fluctuations. In this letter we reveal the nonlinear process capable of channelling the energy of low-frequency electromagnetic to higher-frequency electrostatic fluctuations observed aboard Parker Solar Probe. We present Hall-MHD simulations demonstrating that low-frequency electromagnetic fluctuations can resonate with the ion-sound mode, which results in steepening of plasma density fluctuations, electrostatic spikes and harmonics in the electric field spectrum. The resonance can occur around the wavenumber determined by the ratio between local sound and Alfv\'{e}n speeds, but only in the case of {\it oblique} propagation to the background magnetic field. The resonance wavenumber, its width and steepening time scale are estimated, and all indicate that the revealed two-wave resonance can frequently occur in the solar wind. This process can be a potential channel of energy transfer from cyclotron resonant ions producing the electromagnetic fluctuations to Landau resonant ions and electrons absorbing the energy of the higher-frequency electrostatic fluctuations.