Normalized growth rate of perpendicularly propagating O⁺ cyclotron harmonics plotted against normalized real frequency for varying ambient plasma parameters. Wave propagation angle: θ = 89﹩\mathop{.}\limits^{\unicode{x000b0}}﹩99. (a) Plasma density varied from 100 to 400 cm⁻³ for IMB parameters. (b) Ambient magnetic field strength varied from 50 to 20 nT for IMB parameters. (c) Plasma density varied from 1000 to 700 cm⁻³ for upper ionosphere parameters. Only the fifth harmonic (n = 5) is displayed. (d) Ambient magnetic field strength varied from 10 to 40 nT for upper ionosphere plasma parameters.