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National Aeronautics and Space Administration's Magnetosphere Multiscale mission reveals that agyrotropic electrons and intense waves are prevalently present in the electron diffusion region. Prompted by two distinct Magnetosphere Multiscale observations, this letter investigates by theoretical means and the properties of agyrotropic electron beam-plasma instability and explains the origin of different structures in the wave spectra. The difference is owing to the fact that in one instance, a continuous beam mode is excited, while in the other, discrete Bernstein modes are excited, and the excitation of one mode versus the other depends on physical input parameters, which are consistent with observations. Analyses of dispersion relations show that the growing mode becomes discrete when the maximum growth rate is lower than the electron cyclotron frequency. Making use of particle-in-cell simulations, we found that the broadening angle Delta in the gyroangle space is also an important factor controlling the growth rate. Ramifications of the present finding are also discussed.

Plain Language Summary Magnetospheric Multiscale mission has observed magnetic reconnection process, which converts magnetic energy to kinetic energy of charged particles. Extremely rapid time scale data reveal that electron scale high-frequency waves exist near the electron diffusion region of magnetic reconnection. Recently, two different types of waves observed; one is discrete electron-Bernstein waves, and the other is continuous beam modes. In this study, we formulated a unified theory for both types of waves. Comparing Magnetosphere Multiscale observations, the theory, and particle-in-cell simulations, this study shows that the same cause (agyrotropic electrons) can make two different wave structures depending on plasma parameters. The condition that the maximum growth rate of instabilities equals the electron cyclotron frequency can be considered as a threshold of the transition from discrete electron Bernstein waves to continuous beam modes.