Ratio between the column densities of H₃⁺ levels: (1,0)/(1,1) vs. (3,3)/(1,1) (top); and (3,3)/(2,2) vs. (3,3)/(1,1) (bottom). Symbols as in Fig. 3. The blue diamond corresponds to the excited GC IRS 3 line of sight toward the GC, and the dark blue square in the top panel to diffuse and dense molecular clouds in the Galactic disk. The background grids show the ratios predicted by NLTE models. The green (orange) lines mark the ratios for the temperature, in K, (density, in log cm⁻³) indicated by the colored labels.