O⁺ cyclotron harmonics driven by energetic ring-beam O⁺ ions with E_r = E_b = 100 eV, b = d = 2, and n_s = 0.02n_e: (a)–(c) normalized real frequency plotted against normalized perpendicular wavenumber and (d)–(f) normalized growth rate plotted against normalized real frequency. Plasma regions are indicated at the top of each subplot. Wave propagation angle: ﹩\theta =89.9^\circ ﹩.