Harmonic ECME Excited by Energetic Electrons Travelling Inside A Coronal Loop

TL;DR

This study develops a numerical scheme combining magnetic field extrapolation, guiding-center, and PIC simulations to investigate electron-driven radio emissions in solar coronal loops, revealing mechanisms for solar radio bursts.

Contribution

It introduces a novel integrated numerical approach to connect large-scale magnetic structures with kinetic plasma processes in solar flares.

Findings

VDFs show strip-like and loss-cone features capable of driving ECME.

Harmonic X mode can be excited by strip-like VDF features.

Energy conversion rate to X2 mode can reach ~2.9×10^-3 of initial electron energy.

Abstract

A complete understanding of solar radio bursts requires developing numerical techniques which can connect large-scale activities with kinetic plasma processes. As a starting point, this study presents a numerical scheme combining three different techniques: (1) extrapolation of magnetic field overlying a specific active region in order to derive the background field, (2) guiding-center simulation of dynamics of millions of particles within a selected loop to reveal the integral velocity distribution function (VDF) around certain sections of the loop, and (3) particle-in-cell (PIC) simulation of kinetic instabilities driven by energetic electrons initiated by the obtained distributions. Scattering effects at various levels (weak, moderate, and strong) due to wave/turbulence-particle interaction are considered using prescribed time scales of scattering. It was found that the obtained VDFs contain strip-like and loss-cone features with positive gradient, and both features are capable of driving electron cyclotron maser emission (ECME), which is a viable radiation mechanism for some solar radio bursts, in particular, solar radio spikes. The strip-like feature is important in driving the harmonic X mode, while the loss-cone feature can be important in driving the fundamental X mode. In the weak-scattering case, the rate of energy conversion from energetic electrons to X2 can reach up to ~2.9 * 10^-3 Ek0, where Ek0 is the initial kinetic energy of energetic electrons. The study demonstrates a novel way of exciting X2 mode in the corona during solar flares, and provides new sight into how escaping radiation can be generated within a coronal loop during solar flares.