Electron‐acoustic and cyclotron‐sound instabilities driven by field‐aligned hot‐electron streaming

We study the magnetized electron‐acoustic instability in a three‐component plasma comprising cool ions, cool electrons and a hot, drifting electron component. Whereas previous investigations of the electron‐acoustic instability have usually been limited to the regime of strong magnetization, we have considered strong, intermediate, and weak magnetization regimes. In the weak to intermediate regime we identify an electron‐cyclotron‐sound instability which has lower threshold and larger growth rate than the weakly magnetized electron‐acoustic instability for wave propagation angles quasi‐parallel to the ambient magnetic field. The relationship between the cyclotron‐sound and electron‐acoustic instabilities is elucidated. In particular, we observe a “coupling” of the waves in weak magnetic fields, which becomes weaker and finally disappears with progressively larger magnetic field strengths. The relevance of this work to cusp auroral hiss and broadband electrostatic noise in the auroral magnetosphere is discussed.