Harmonic elctron-cyclotron maser emissions driven by energetic electrons of the horseshoe distribution with application to solar radio spikes

TL;DR

This study uses kinetic simulations to demonstrate that horseshoe electron distributions can generate harmonic electron-cyclotron maser emissions, potentially explaining solar radio spikes and overcoming previous escape difficulties.

Contribution

It introduces a novel simulation approach showing horseshoe-driven ECME efficiently produces harmonic emissions, addressing the escaping problem in solar radio spike models.

Findings

Efficient amplification of X2 and X3 modes observed.

X2 emission intensity increases with higher electron density ratio.

Energy conversion to X2 modes reaches up to 0.17% at 10% density ratio.

Abstract

Content. Electron-cyclotron maser emission (ECME) is the favored mechanism for solar radio spikes and has been investigated extensively since the 1980s. Most studies relevant to solar spikes employ a loss-cone-type distribution of energetic electrons, generating waves mainly in the fundamental X/O mode (X1/O1), with a ratio of plasma oscillation frequency to electron gyrofrequency (${\omega}_ {pe}/{\Omega}_{ce}$) lower than 1. Despite the great progress made in this theory, one major problem is how the fundamental emissions pass through the second-harmonic absorption layer in the corona and escape. This is generally known as the escaping difficulty of the theory. Aims. We study the harmonic emissions generated by ECME driven by energetic electrons with the horseshoe distribution to solve the escaping difficulty of ECME for solar spikes. Methods. We performed a fully kinetic electromagnetic PIC simulation with ${\omega}_ {pe}/{\Omega}_{ce}$ = 0.1, corresponding to the strongly magnetized plasma conditions in the flare region, with energetic electrons characterized by the horseshoe distribution. We also varied the density ratio of energetic electrons to total electrons ($n_e/n_0$) in the simulation. Results. We obtain efficient amplification of waves in Z and X2 modes, with a relatively weak growth of O1 and X3. With a higher-density ratio, the X2 emission becomes more intense, and the rate of energy conversion from energetic electrons into X2 modes can reach $\sim$0.06% and 0.17%, with $n_e/n_0$= 5% and 10%, respectively. Conclusions. We find that the horseshoe-driven ECME can lead to an efficient excitation of X2 and X3 with a low value of ${\omega}_ {pe}/{\Omega}_{ce}$, providing novel means for resolving the escaping difficulty of ECME when applied to solar radio spikes. The simultaneous growth of X2 and X3 can be used to explain some harmonic structures observed in solar spikes.