Contours of the toroidal magnetic field (white lines) and of the growth rate of the instability (coloured contours) for the most unstable perturbation (m = 7 for the top frame and m = 1 for the bottom frame) which is in the direction e = e_ϖ. The parameters of the simulations are N = 0 and, respectively for the top and the bottom frames Ω /ω_Aϕ ≈ 10 and Ω /ω_Aϕ ≈ 1 at the maximum of the toroidal field.