Top panel: transverse (self-generated) component of B for a M = 20 parallel shock at (Run B). Bottom panel: power spectrum of B_⊥ as a function of wavenumber k. The color code matches corresponding shock regions. The vertical dashed and dot-dashed line indicate modes resonant with ions of energy E_sh and E_max ~ 300E_sh, respectively. Symbols correspond to , i.e., the spectral energy distribution produced by a ∝p⁻⁴ CR distribution via resonant streaming instability.