Amplitude spectra of the magnetospheric coefficient |$q_{21}^\mathrm{mag}$| (upper panel), the ionospheric coefficient |$q_{21}^\mathrm{ion}$| coefficient (middle panel) and the internally induced coefficient |$g_{21}^\mathrm{int}$| (lower panel) around the diurnal band. Spectral peaks observed in the spectra are marked with triangles, with their peak periods annotated. Selected theoretical periods of the system, including the diurnal period and its harmonics, and the periods for lunar daily variations |$L_p$| with |$p=1,2,3,4$| are plotted as solid vertical lines for reference.