Lunar Variations in Auroral Zone

Abstract

AN investigation of the frequency content of the geomagnetic record, collected during the IGY/IGC at several stations spread over the globe, has been carried out by high resolution spectrum analysis. I have already described the method¹ and a detailed discussion of the analysis will appear elsewhere. At most of the stations, besides the solar harmonics, several lunar tidal lines and luni-solar lines appeared significantly above the noise level of the spectrum. The amplitudes of these lines have been estimated from the power energy of the appropriate peak in the spectrum; in this note only the variation of the amplitude of the harmonic terms L₁, L₂ and L₃ with respect to the dipole latitude is considered. The frequencies of these lines as calculated by Chapman's formula are 0.93227, 1.93227 and 2.93227 cycles per day respectively and the frequencies of the corresponding lunar tide lines O₁, M₂ and M′₃ (corresponding to the term A₈₄ of Schureman³) are respectively 0.92953, 1.93227 and 2.93427 cycles per day. With the frequency interval (0.00416666 cycles per day) allowed for in the spectrum analysis, the minor differences in the frequencies of the luni-solar and the tidal lines remain indistinguishable and the resultant effect appears as a peak in the spectrum.