Observation of an Extraordinary Type V Solar Radio Burst: Nonlinear Evolution of the Electron Two-Stream Instability

TL;DR

This paper reports the observation of a unique, circularly polarized type V solar radio burst linked to electron two-stream instability and nonlinear electron firehose instability, offering new insights into solar radio emission mechanisms.

Contribution

It presents the first observation of a type V burst associated with nonlinear electron firehose instability, expanding understanding of solar radio burst processes.

Findings

Type V burst is circularly polarized, unlike U bursts.

The burst's frequency drift is slower than associated U bursts.

Electron firehose instability influences the emission process.

Abstract

Solar type V radio bursts are associated with type III bursts. Several processes have been proposed to interpret the association, electron distribution, and emission. We present the observation of a unique type V event observed by e-CALLISTO on 7 May 2021. The type V radio emission follows a group of U bursts. Unlike the unpolarized U bursts, the type V burst is circularly polarized, leaving room for a different emission process. Its starting edge drifts to higher frequency four times slower than the descending branch of the associated U burst. The type V processes seem to be ruled by electrons of lower energy. The observations conform to a coherent scenario where a dense electron beam drives the two-stream instability (causing type III emission) and, in the nonlinear stage, becomes unstable to another instability, previously known as the electron firehose instability (EFI). The secondary instability scatters some beam electrons into velocities perpendicular to the magnetic field and produces, after particle loss, a trapped distribution prone to electron cyclotron masering (ECM). A reduction in beaming and the formation of an isotropic halo are predicted for electron beams continuing to interplanetary space, possibly observable by Parker Solar Probe and Solar Orbiter.