# Wave excitation by energetic ring-distributed electron beams in the solar corona

## TL;DR

This study uses kinetic simulations to investigate wave excitation by energetic ring-distributed electron beams in the solar corona, revealing conditions for electromagnetic wave escape and implications for solar radio burst observations.

## Contribution

It provides the first detailed analysis of wave excitation by ring-beam electrons in the solar corona using self-consistent kinetic simulations, highlighting the role of plasma magnetization and electron density ratios.

## Key findings

- Electrostatic waves dominate wave energetics in the corona.
- Escaping electromagnetic waves are effectively excited in strongly magnetized plasmas.
- Wave polarization varies with ring-beam electron density, explaining solar radio burst properties.

## Abstract

We analyzed properties of waves excited by mildly relativistic electron beams propagating along magnetic field with a ring-shape perpendicular momentum distribution in neutral and current-free solar coronal plasmas. These plasmas are subject to both the beam and the electron cyclotron maser (ECM) instabilities driven by the positive momentum gradient of the ring-beam electron distribution in the directions parallel and perpendicular to the ambient magnetic field, respectively. To explore the related kinetic processes self-consistently, 2.5-dimensional fully kinetic particle-in-cell (PIC) simulations were carried out. To quantify excited wave properties in different coronal conditions, we investigated the dependence of their energy and polarization on the ring-beam electron density and magnetic field. In general, electrostatic waves dominate the energetics of waves and nonlinear waves are ubiquitous. In weakly magnetized plasmas, where the electron cyclotron frequency $\omega_{ce}$ is lower than the electron plasma frequency $\omega_{pe}$, it is difficult to produce escaping electromagnetic waves with frequency $\omega > \omega_{pe}$ and small refractive index $|c k / \omega| < 1$ ($k$ and $c$ are the wavenumber and the light speed, respectively). Highly polarized and anisotropic escaping electromagnetic waves can, however, be effectively excited in strongly magnetized plasmas with $\omega_{ce}/\omega_{pe} \geq 1$. The anisotropy of the energy, circular polarization degree (CPD), and spectrogram of these escaping electromagnetic waves strongly depend on the number density ratio of the ring-beam electrons to the background electrons. In particular, their CPDs can vary from left-handed to right-handed with the decrease of the ring-beam density, which may explain some observed properties of solar radio bursts (e.g., radio spikes) from the solar corona.