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Electromagnetic radiation at higher harmonics of the plasma frequency($\omega \sim n\omega_{pe}, n > 2$) has been occasionally observed in type IIand type III solar radio bursts, yet the underlying mechanism remainsundetermined. Here we present two-dimensional fully kinetic electromagneticparticle-in-cell simulations with high spectral resolution to investigate thebeam-driven plasma emission process in weakly magnetized plasmas of typicalcoronal conditions. We focused on the generation mechanisms of high-harmonicemission. We found that a larger beam velocity ($u_d$) favors the generation ofthe higher-harmonic emission. The emissions grow later for higher harmonics anddecrease in intensity by $\sim$2 orders of magnitude for each jump of theharmonic number. The second and third harmonic ($\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., $\rm H_2 + L \rightarrow H_3$, whereinthe coalescence with the forward-propagating beam-Langmuir wave leads to theforward-propagating $\rm H_3$, and coalescence with the backward-propagatingLangmuir wave leads to the backward-propagating $\rm H_3$; and (2) the $\rmH_4$ emission mainly arises from the coalescence of the $\rm H_3$ emission withthe forward- (backward-) propagating Langmuir wave, in terms of $\rm H_3 + L\rightarrow H_4$.

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