Electromagnetic ion beam instabilities: II

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Abstract
This paper extends the results of Gary et al. [Phys. Fluids 2 7, 1852 (1984)]. That paper examined the linear theory of electromagnetic instabilities driven by an ion beam streaming along a magnetic field in a homogeneous Vlasov plasma, emphasizing the parametric dependence of the instability growth rates. This paper considers the parametric dependence of the real frequency at maximum growth of two such instabilities with right‐hand polarization. It is shown that the right‐hand resonant ion beam instability can have maximum growth at frequencies near the ion‐cyclotron frequency if the beam–main component relative drift speed is about twice the Alfvén speed and at least one of the following conditions holds: the ion beta or the beam–main component relative temperature are sufficiently small, or the perpendicular‐to‐parallel beam temperature ratio is sufficiently large. These results support the identification of the right‐hand resonant instability as the source of the large amplitude magnetic fluctuations observed upstream of slow shocks in the Earth’s magnetotail.