The dynamics of the solar magnetic field: polarity reversals, butterfly diagram and quasi-biennial oscillations

The spatio-temporal dynamics of the solar magnetic field has been investigated by using NSO/Kitt Peak synoptic magnetic maps covering the period August 1976-September 2003. For each heliographic latitude the field has been decomposed in intrinsic mode functions through the Empirical Mode Decomposition, in order to investigate the time evolution of the various characteristic oscillating frequencies at different latitudes. The quasi biennial oscillations are identified as the fundamental periodicity of the magnetic field and linked to dynamo waves which transport magnetic flux both polewards and equatorwards from latitudes of about 35°. On the other hand, the usual pattern of the Sporer law is associated with periodicities related to the high frequency fluctuating part of the magnetic field. Finally the usual ~22 yr cycle, related to the polarity inversions of the large-scale dipolar field, are consistent with alpha-omega dynamo models including meridional circulation.