Resonance broadening due to particle scattering and mode-coupling in the quasi-linear relaxation of electron beams

TL;DR

This paper develops a resonance-broadened weak-turbulence theory incorporating mode-coupling to ion-sound modes, explaining how scattering processes can suppress electron beam instabilities in turbulent plasmas relevant to solar flare diagnostics.

Contribution

It introduces a unified resonance-broadening framework accounting for wave and particle scattering, advancing understanding of beam relaxation in turbulent plasma environments.

Findings

Resonance broadening can suppress beam instabilities.

Mode-coupling to ion-sound modes affects wave-particle interactions.

The theory explains observed stability of electron populations in solar wind.

Abstract

Of particular interest for radio and hard X-ray diagnostics of accelerated electrons during solar flares is the understanding of the basic non-linear mechanisms regulating the relaxation of electron beams propagating in turbulent plasmas. In this work, it is shown that in addition to scattering of beam electrons, scattering of the beam-generated Langmuir waves via for instance mode-coupling, can also result in broadening of the wave-particle resonance. We obtain a resonance-broadened version of weak-turbulence theory with mode-coupling to ion-sound modes. Resonance broadening is presented here as a unified framework which can quantitatively account for the reduction and possible suppression of the beam instability due to background scattering of the beam electrons themselves or due to scattering of the beam-generated Langmuir waves in fluctuating plasmas. Resonance broadening being essentially equivalent to smoothing of the electron phase-space distribution, it is used to construct an intuitive physical picture for the stability of inverted populations of fast electrons that are commonly observed \emph{in-situ} to propagate in the solar-wind.