Marginal stability of oblique whistler-mode waves

The problem of marginal stability of oblique whistler-mode waves propagating in a hot, anisotropic plasma is considered both within the quasilongitudinal and quasielectrostatic approximations. It is pointed out that the most favourable region for quasilongitudinal whistler-mode instability lies at frequencies ω above the half electron gyrofrequency Ω unless the wave normal angle is very close to zero. In the opposite limiting case, the waves are expected to be excited at ω < Ω/2. This can provide a possible explanation of a gap at ω ∼ Ω/2 in the spectrum of chorus emissions observed in the magnetosphere. Quasielectrostatic whistler-mode instabilities are expected for values of anisotropy which seem to be too large for magnetospheric conditions.