Time profiles of the magnetic field in the stream for 2D and 3D simulations (dotted and solid lines, respectively). Simulations are shown for n_s = 10⁻² and 10⁻³ cm⁻³ in purple and orange, respectively. The dotted grey line stands for a 2D simulation with n_s = 10⁻² cm⁻³ and with a magnetic field parallel to the stream (no amplification due to stretching). The dashed lines show the model results from Eq. (2) with the same Mach number as the simulations (ℳ = 1).