The instability of the ordinary electromagnetic mode, propagating perpendicular to an external magnetic field, is studied for a single-species plasma with ring velocity distribution. The marginal instability boundaries for both the purely growing mode and the propagating growing modes are calculated from the instability criteria. The dispersion characteristics for various sets of plasma parameters are also given. The frequency bands of the propagating growing modes are centered at about (n + 1/2) ωc, where n = 1, 2, and the typical growth rates are of the order of the cyclotron frequency and enhanced by increasing β.
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