High-harmonic Plasma Emission Induced by Electron Beams in Weakly Magnetized Plasmas

Electromagnetic radiation at higher harmonics of the plasma frequency ($\omega \sim n\omega_{pe}, n > 2$) has been occasionally observed in type II and type III solar radio bursts, yet the underlying mechanism remains undetermined. Here we present two-dimensional fully kinetic electromagnetic particle-in-cell simulations with high spectral resolution to investigate the beam-driven plasma emission process in weakly magnetized plasmas of typical coronal conditions. We focused on the generation mechanisms of high-harmonic emission. We found that a larger beam velocity ($u_d$) favors the generation of the higher-harmonic emission. The emissions grow later for higher harmonics and decrease in intensity by $\sim$2 orders of magnitude for each jump of the harmonic number. The second and third harmonic ($\rm H_2$ and $\rm H_3$) emissions get closer in intensity with larger $u_d$. We also show that (1) the $\rm H_3$ emission is mainly generated via the coalescence of the $\rm H_2$ emission with the Langmuir waves, i.e., $\rm H_2 + L \rightarrow H_3$, wherein the coalescence with the forward-propagating beam-Langmuir wave leads to the forward-propagating $\rm H_3$, and coalescence with the backward-propagating Langmuir wave leads to the backward-propagating $\rm H_3$; and (2) the $\rm H_4$ emission mainly arises from the coalescence of the $\rm H_3$ emission with the forward- (backward-) propagating Langmuir wave, in terms of $\rm H_3 + L \rightarrow H_4$.