Magnetosonic RDI in MHD: blue (orange) lines show numerically calculated RDI growth rates for grains resonant with the fast (slow) mode with streaming angle ﹩{\theta }_{w}=45^\circ ﹩ and μ = 0.01 (μ = 0.05 for the ﹩{w}_{s}=10{c}_{s}﹩ fast wave). Dotted lines show the analytic predictions (15) (Equation (12) at high k). In each case, we calculate ζs and ζw assuming Epstein drag (9) with γ = 5/3. The resonant mode directions, ﹩\hat{{\boldsymbol{k}}}=(\cos \phi \sin \theta ,\sin \phi \sin \theta ,\cos \theta )﹩, are θ = 70°, ϕ = 49.°6 (slow wave, ﹩{w}_{s}=0.1{c}_{s}﹩); θ = 40°, ϕ = 108.°4 (slow wave, ﹩{w}_{s}=1.5{c}_{s}﹩); θ = 20°, ϕ = 63.°5 (fast wave, ﹩{w}_{s}=1.5{c}_{s}﹩); θ = 30°, ϕ = 57.°6 (fast wave, ﹩{w}_{s}=10{c}_{s}﹩). The low-k discrepancy of the fast-wave prediction at ﹩{w}_{s}=10{c}_{s}﹩ is due to an additional instability.