Top panel: magnetic field upstream of the shock at , for different Mach numbers as in the legend. B_tot(x) is averaged over 200 c/ω_p in the transverse size and over in time, in order to smooth time and space fluctuations. Bottom panel: total amplification factor, averaged over a distance Δx = 10 M_Ac/ω_p ahead of the shock, as a function of the Alfvénic Mach number (red symbols). The dashed line corresponds to B_tot/B₀² = 0.45 M_A, and represents the prediction of resonant streaming instability (see Equation (2), with ζ_cr = 0.15).