Harmonic maser emissions from electrons with loss-cone distribution in solar active regions

TL;DR

This study uses kinetic simulations to explore how energetic electrons in solar active regions generate harmonic radio emissions through nonlinear processes, providing insights into the ECME mechanism.

Contribution

It introduces a new nonlinear coalescence mechanism for harmonic emissions in low plasma-to-gyrofrequency ratio conditions, advancing understanding of solar radio bursts.

Findings

Strong second-harmonic X mode emissions observed

Harmonic emissions produced by nonlinear coalescence processes

Mechanism may explain solar radio emission escape difficulties

Abstract

Electron cyclotron maser emission (ECME) is regarded as a plausible source for the coherent radio radiations from solar active regions (e.g., solar radio spikes). In this Letter, we present a 2D3V fully kinetic electromagnetic particle-in-cell (PIC) simulation to investigate the wave excitations and subsequent nonlinear processes induced by the energetic electrons in the loss-cone distribution. The ratio of the plasma frequency to the electron gyrofrequency ${\omega}_{pe}/{\Omega}_{ce}$ is set to 0.25, adequate for solar active region conditions. As a main result, we obtain strong emissions at the second-harmonic X mode (X2). While the fundamental X mode (X1) and the Z mode are amplified directly via the electron cyclotron maser instability, the X2 emissions can be produced by the nonlinear coalescence between two Z modes and between Z and X1 modes. This represents a novel generation mechanism for the harmonic emissions in plasmas with a low value of ${\omega}_{pe}/{\Omega}_{ce}$, which may resolve the escaping difficulty of explaining solar radio emissions with the ECME mechanism.