Left: time-evolution of , the normalised projection of B_z onto the prominent axisymmetric Fourier mode (k_x = 2π/L_x) in the ideal simulation with μ_eq = 10⁻³. Right: growth rate of the linear instability as a function of the magnetisation μ. The red diamond markers are measures from 3D ideal simulations. The green and yellow crosses are those measured in resistive and ambipolar runs. The plain lines correspond to model predictions, using similar scaling relations as (17), but with η_t = 0.5α_ν in the ideal case.