Evolution of the typical growth time T_growth = 1/σ of the instability in Io with the tidal bulge ellipticity β and the libration amplitude ϵ (α = 2.62). The upper right corner corresponds to the optimal case considered in Kerswell & Malkus (1998) and Herreman et al. (2009), the upper left corner corresponds to optical librations with a small tidal amplitude corresponding to diurnal tides (β = 7 × 10^-5,  ϵ = 2e = 0.0082) and the lower right corner corresponds to the region of physical librations of the static bulge (β = 0.006,  ϵ = 1.3 × 10^-4). The white zone corresponds to the stable zone where the instability cannot grow because of dissipative effects (at the boundary with the colored zone, the growth time is infinite). The colorbar range is chosen so that color variations are visible.