Geometry of the problem. We assume that the x, y plane separates two regions, this is the interface. Each region contains a density ρ₁ (for the lower region) and ρ₂ (for the upper region) with ρ₁ < ρ₂. The entire system is immersed into a magnetic field B lying in the x, z plane, forming an angle θ with the x-axis. We assume that the perturbation develops along the wave vector k, which lies in the x, y plane and forms an angle α with the x-axis. The charged (resp. neutral) fluid is subject to the gravity field g_c (resp. g_n), directed along −e_z.