Normalized growth rate of perpendicularly propagating O⁺ cyclotron harmonics plotted against normalized real frequency for varying cold background plasma composition. Wave propagation angle: θ = 89﹩\mathop{.}\limits^{\unicode{x000b0}}﹩99. (a) Cold proton density is varied with ﹩{n}_{{{\rm{O}}}^{+}}:{n}_{{{\rm{O}}}_{2}^{+}}=2:1﹩ for IMB parameters. Green curve shows the case where the background ions are completely replaced by H⁺. (b) Cold heavier ion concentration is varied with ﹩{n}_{{{\rm{O}}}^{+}}+{n}_{{{\rm{O}}}_{2}^{+}}=0.18{n}_{e}﹩ for IMB parameters. (c) Same as panel (b) but with an increased plasma density of 600 cm⁻³. (d) Cold heavier ion concentration is varied with ﹩{n}_{{{\rm{O}}}^{+}}+{n}_{{{\rm{O}}}_{2}^{+}}=0.96{n}_{e}﹩ for upper ionosphere parameters. Only the third harmonic (n = 3) is shown.