Theory and simulation of electromagnetic beam modes and whistlers

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Abstract
Using particle‐in‐cell simulations and analytical methods, a study of the nonlinear evolution of electromagnetic instabilities driven by an anisotropic electron beam (Tb⊥≫Tb∥) in an external magnetic field is performed. The unstable waves are either whistlerlike or beam‐mode‐like depending on the external field strength and beam velocity. The evolution of the particle distribution differs significantly in the two regimes. Even in the presence of a faster electrostatic instability, the electromagnetic waves grow to a significant amplitude. In certain cases, an energetic tail is formed, resulting in enhanced Čerenkov emission of electrostatic waves. The initial evolution of the particle distribution is explained in terms of the interaction of a given linearly unstable wave with the self‐consistent perturbed distribution.