Linear growth rate of the RTI as a function of the wavenumber. The values chosen for the plot are ρ₂ = 10^-10 kg m^-3, ρ₂/ρ₁ = 100, θ = 85° m^-1, B₀ = 10 G, c_s2 = 15 km s^-1, ξ₂ = 0.5, and ξ₁ = 0.1. The dashed line is the incompressible MHD limit from Eq. (1); the dotted line corresponds to the PI one-fluid model with only the ambipolar term (Sect. 4); and the solid line to the PI one-fluid model with all the terms in the induction equation.