Eigenvalue spectra of the unforced problem (ϵ = 0) without (N ₀ = 0; upper panels) and with (N ₀ = 100; lower panels) stratification. Panels (b) and (d) display the damping, ﹩\mathrm{Im}[\omega ]﹩, versus the frequency, ﹩\mathrm{Re}[\omega ]﹩, of the least-damped eigensolutions near the forcing frequency (ω = 0.67) of Mercury’s longitudinal libration, denoted by the vertical black line. The eigensolution with the frequency closest to this libration frequency is marked in both panels with a black square, and meridional cuts of their corresponding kinetic energy density are shown in panels (a) and (c).