Plasma Emission Induced by Electron Cyclotron Maser Instability in Solar Plasmas with a Large Ratio of Plasma Frequency to Gyrofrequency

In plasmas with a large ratio of plasma frequency to gyrofrequency (ωpe/Ωce), energetic electrons characterized by $\partial f/\partial {v}_{\perp }\gt 0$ can excite electron cyclotron maser instability (ECMI), generating waves of upper hybrid (UH), Z, and W modes. It has been presumed that these ECMI waves can somehow convert to escaping X–O modes as fundamental (F) or harmonic (H) plasma emission. Here we perform a fully kinetic, electromagnetic particle-in-cell simulation to investigate the proposed radiation process. ECMI is driven by energetic electrons with a Dory–Guest–Harris distribution representative of a double-sided loss cone, and ωpe/Ωce is set to be 10. We find that the electrostatic UH mode is the fastest-growing mode. Around the time when its energy starts to decline, the W mode grows to be dominant. During this stage, we observe significant F and H plasma emission. The F emission is in the O mode with a bandwidth around 0.1–0.2 Ωce, and the H emission is contributed by both X and O modes with a narrower bandwidth. We suggest that the O–F emission is caused by coalescence of almost counterpropagating Z and W modes, while the H emission arises from coalescence of an almost counterpropagating UH mode at relatively large wave number. Thus the plasma emission investigated here is induced by a combination of wave growth due to ECMI and further nonlinear wave-coupling processes. The result is relevant to understanding solar radio bursts as well as other astronomical radio sources that are excited by energetic electrons trapped within certain magnetic structures.