A modulation model for the solar and lunar daily geomagnetic variations
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences. 2003
Received: 9 December 2002
Accepted: 25 July 2003
Published: 20 June 2014
The traditional Chapman-Miller analysis for the solar, S, and lunar, L, geomagnetic variations is generalized by incorporating a time description of the seasonal changes of the harmonic coefficients. The modulation model consists of the sum of harmonic oscillators with basic carriers having the fundamental frequencies of the solar and lunar daily components, which are being amplitude and phase modulated by the annual variation and its harmonics. The solar cycle effect is a priori taken into account by using the daily sunspot numbers as an auxiliary input and including the Wolf ratios in the amplitude and phase terms. For the station Dourbes (Belgium), solar and lunar harmonics show a marked increase in amplitude from winter to summer, but the seasonal changes of L significantly exceed that of S. The phase shift from winter to summer in L is about three times that of S. The Wolf ratios of the Fourier amplitudes are of the same order of magnitude for both the S and L variations. Removal of the relatively important ocean dynamo contribution does not have an appreciable consequence for the determination of the seasonal changes and the sunspot cycle influence.