Number of unstable modes as a function of the Ekman number at different radii for m = 1, 2, 3. For m ≥ 4, all modes are stable. The solar differential rotation is from Larson & Schou (2018). The dashed-black line gives the Ekman number E_MLT as a function of the radius using the turbulent viscosity from mixing-length theory (Muñoz-Jaramillo et al. 2011). The solid black line shows the Ekman number E_η, using the quenched diffusivity model proposed by Muñoz-Jaramillo et al. (2011), see their Fig. 4b. The regions corresponding to the most unstable modes (R₂, R₁, and R) and to the two most unstable modes (R₂ and R₁, R₁ and R₂, and R and R₁) are colored and labeled with these modes. The color bar indicates the total number of unstable modes at any point in the diagram.