Density bumps at selected snapshots, for the MHD simulations (upper panels) and alpha-disk equivalents (lower panels). The dashed lines bracket 2H around the density maximum of the last snapshot shown (red line). The RWI “rule-of-thumb” of 10−20% variation in density over a width comparable to the scale height is satisfied for the MHD turbulent runs, even for the wide transition model of h₁ = 0.8. In the equivalent viscous laminar model the density bumps diffuse over a much larger width, and the RWI is not excited.