LUNAR DAILY HARMONIC GEOMAGNETIC VARIATION AS INDICATED BY SPECTRAL ANALYSIS

Two series of hourly magnetic data, one from 54 observatories, have been analyzed spectrally and also, in some cases, harmonically. The main series is for the sunspot maximum 1957-1959, the other for the preceding minimum, 1953-1955. In all, 210 annual sets of hourly values, chiefly for the horizontal component H, were used. Prominent peaks appear in the spectra for the harmonics Sn (with frequencies n cycles/day) of the solar daily variation S, for n up to 4, and in a few cases even up to 11. Lower peaks are shown for the components Ln of the lunar daily variation L, up to 4, caused by the dynamo action, in the ionosphere, of the main term M2 in the lunar atmospheric tide; the term T1 possibly includes a significant part caused by the next main lunar tidal term, O1 (diurnal). Other peaks appear less prominently, that may be identifiable with the L harmonics corresponding to dynamo action of minor lunar tidal terms; the indications of these peaks should be checked by harmonic analysis of longer series of data. For some of the data sets the amplitudes inferred from the spectral analysis are compared with those derived by harmonic analysis. The latter are systematically smaller, but for the solar harmonics Sn the difference is slight. It is large, however, for the main lunar harmonics derived from high latitude data, which show much irregular disturbance. © 1969.