Linear growth rate of the RTI for a partiallyl ionized plasma (taking into account ambipolar diffusion) as a function of the Alfvén speed . The dashed line corresponds to the incompressible limit given in Eq. (1) and the black solid line to the compressible MHD results from Sect. 4.3. The values ρ₂/ρ₁ = 100, β = 0.1, θ = 40, k = 10^-7 m^-1, c_crit = 33 km s^-1, and ξ_n1 = 10^-4 have been used.