Large Scale Solar Magnetic Fields: Temporal Variations

We have investigated the temporal evolution of the solar magnetic field during solar cycles 20, 21 and 22 by means of spherical harmonic decomposition and subsequent time series analysis. A 33 yr and a 25 yr time series of daily magnetic maps of the solar photosphere, recorded at the Mt. Wilson and NSO/Kitt Peak observatories respectively, were used to calculate the spherical coefficients of the radial magnetic field. Fourier and wavelet analysis were then applied to deduce the temporal variations. We compare the results of the two datasets and present examples of zonal modes which show significant variations, e. g. with a period of approx. 2.0—2.5 years. We provide evidence that this quasi-biennial oscillation originates mainly from the southern hemisphere. Furthermore, we show that low degree modes with odd l — m exhibit periods of 29.2 and 28.1 days while modes with even l — m show a dominant period of 26.9 days. A resonant modal structure of the solar magnetic field (apart from the 22 yr cycle) has not been found.