Gap formation around Ωe/2 and generation of low‐band whistler waves by Landau‐resonant electrons in the magnetosphere: Predictions from dispersion theory

In this paper we show that two significant phenomena of magnetospheric chorus emission can be explained by the participation of beam‐like electron structures, created by Landau‐resonant interaction with growing oblique whistler waves. The first concerns the widely observed spectral gap near half the electron cyclotron frequency Ω e ; the second is related to the observation of very obliquely propagating lower‐band waves that cannot be directly generated by temperature anisotropy. Concerning the gap, kinetic dispersion theory reveals that interference of the beam‐related cyclotron mode ω~Ω e ‐kV b with the conventional whistler mode leads to mode splitting and the appearance of a ‘forbidden’ area in the ω‐k space. Thereby the beam velocity V b appears as an essential parameter. It is directly related to the phase velocity of the most unstable whistler wave mode, which is close to V Ae /2 for sufficiently hot electrons ( V Ae is the electron Alfven velocity). To clarify the second point, we show that Landau‐resonant beams with V b < V Ae /2, which arise in cold plasmas from unstable upper‐band waves, are able to generate lower‐band whistler mode waves at very oblique propagation ( θ ≥ 60°). Our studies demonstrate the important role of Landau‐resonant electrons in nonlinear whistler wave generation in the magnetosphere.