Various amplitude spectral analyses are made from the 103-year series of 3-hour aa indices and of K indices of the two antipodal observatories from which aa indices are derived. Identical spectra are obtained when one uses the same temporal series as those analyzed by Fraser-Smith (1972) for the Ap indices: only the relative amplitudes of the lines vary. The temporal length of our series allows us to check easily the stability and the significance of various lines. Some are perfectly stable (semiannual variation, annual variation). Others are more or less stable (11-year cycle, 90-year cycle). No significant line corresponds to the 27-day sun rotation, but there exist only clusters of unstable lines associated with an increase of the background noise: the average speed rotation of the recurrent emitting sources (sunspots+ M regions) would be 27.1 days. No influence of the moon is detected. Apart from the annual line and the 1-day line, all the lines (significant or not) are in phase between both hemispheres. A distinction is proposed between astronomical (or 'modulation') lines and astrophysical (or 'excitation') lines.