  It can be shown on observational grounds that two basic effects of dynamo
theory for solar activity - production of the toroidal field from the poloidal
one by differential rotation and reverse conversion of the toroidal field to
the poloidal configuration by helical motions - are operating in the Sun. These
two effects, however, do not suffice for constructing a realistic model for the
solar dynamo. Only when a non-local version of the alpha-effect is applied, is
downward diamagnetic pumping included and field advection by the equatorward
meridional flow near the base of the convection zone allowed for, can the
observed activity cycles be closely reproduced. Fluctuations in the
alpha-effect can be estimated from sunspot data. Dynamo models with fluctuating
parameters reproduce irregularities of solar cycles including the grand
activity minima. The physics of parametric excitation of irregularities
remains, however, to be understood.
