Solar plasma radio emission in the presence of imbalanced turbulence of kinetic-scale Alfv\'en waves

TL;DR

This paper presents a model explaining high-brightness solar type I radio storms through imbalanced turbulence of kinetic-scale Alfvén waves, which suppresses Landau damping and enhances Langmuir wave growth, accounting for observed emission properties.

Contribution

The study introduces a novel model linking imbalanced Alfvén wave turbulence to the generation of high-brightness type I radio emissions in the solar corona.

Findings

Langmuir wave amplitudes grow spontaneously above thermal levels.

The model predicts 100% polarization in the o-mode.

Harmonic emission is suppressed due to Landau damping of backward waves.

Abstract

We study the influence of kinetic-scale Alfv\'enic turbulence on the generation of plasma radio emission in the solar coronal regions where the plasma/magnetic pressure ratio $\beta $ is smaller than the electron/ion mass ratio $m_{e}/m_{i}$. The present study is motivated by the phenomenon of solar type I radio storms associated with the strong magnetic field of active regions. The measured brightness temperature of the type I storms can be up to $10^{10}$ K for continuum emission, and can exceed $10^{11}$ K for type I bursts. At present, there is no generally accepted theory explaining such high brightness temperatures and some other properties of the type I storms. We propose the model with the imbalanced turbulence of kinetic-scale Alfv\'en waves producing an asymmetric quasilinear plateau on the upward half of the electron velocity distribution. The Landau damping of resonant Langmuir waves is suppressed and their amplitudes grow spontaneously above the thermal level. The estimated saturation level of Langmuir waves is high enough to generate observed type I radio emission at the fundamental plasma frequency. Harmonic emission does not appear in our model because the backward-propagating Langmuir waves undergo a strong Landau damping. Our model predicts $100\%$ polarization in the sense of the ordinary (o-) mode of type I emission.