Plasma instability in the front of ejected energetic electrons and Type III solar radiobursts

TL;DR

This paper proposes a revised model for Type III solar radio bursts, suggesting that electron distribution truncation due to the time-of-flight effect, rather than the two-stream instability, drives Langmuir wave generation, aligning well with observations.

Contribution

It introduces a new linear instability mechanism based on electron distribution truncation, challenging the traditional two-stream instability model for Type III bursts.

Findings

Wave intensity growth matches observed burst profiles

Truncation instability can generate Langmuir waves

Model aligns with in-situ spacecraft observations

Abstract

Type III radio bursts are a signature of the flux of near-relativistic electrons ejected during solar flares. These bursts are frequently observed by spacecraft such as the Parker Solar Probe. It is traditionally believed that these electron beams generate Langmuir waves through the two-stream instability, which are then converted into electromagnetic waves. In this study, we revise that model by examining how the electron distribution becomes truncated due to the "time-of-flight" effect as the beam travels through a randomly inhomogeneous, and gently varying solar-wind plasma. Rather than the two-stream instability, this truncation destabilizes the distribution and leads to the generation of Langmuir waves via a linear instability; we confine our analysis to this linear regime and do not take into account the back reaction of the generated Langmuir waves on the electron distribution, which is nonlinear. The instability grows until slower electrons arrive and dampen the waves. Our qualitative analysis shows that the resulting wave intensity growth and decay closely match the intensity-time profile of observed Type III radio bursts at the fundamental frequency, supporting this modified theory.