Growth rate γ as a function of the Reynolds numbers Re and Rm (note that the color scale is different for positive and negative values of γ), in the case of the weakly nonlocal Sabra model (α =  −5/2). The stability curve, corresponding to the γ = 0 level line, is overplotted. The dashed rectangle corresponds to the range of parameters explored by the direct numerical simulations of Iskakov et al. (2007); the green triangles indicate the initial Reynolds numbers used in Fig. 4; the dotted line is where Pm = 1.