Linear growth rate vs. equilibrium magnetic field for ρ₂/ρ₁ = 100, g = 270 m s^-2, θ = 87°, k = 5 × 10^-6 m^-1, ξ_n1 = 10^-4, and ρ₂ = 10^-10 kg m^-3. The dotted line is the ambipolar case with ξ_n2 = 0.5, the solid line with ξ_n2 = 0.05 and the dot-dashed line to ξ_n2 = 0.5 but with a value for the ambipolar diffusion coefficient that is 1000 times larger than the theoretical value used in the rest of the computations. The dashed line corresponds to the incompressible MHD limit (Eq. (1)).