The emergence and growth of the flux transport dynamo model of the sunspot cycle

The sunspot cycle is the magnetic cycle of the Sun produced by the dynamo process. A central idea of the solar dynamo is that the toroidal and the poloidal magnetic fields of the Sun sustain each other. We discuss the relevant observational data both for sunspots (which are manifestations of the toroidal field) and for the poloidal field of the Sun. We point out how the differential rotation of the Sun stretches out the poloidal field to produce the toroidal field primarily at the bottom of the convection zone, from where parts of this toroidal field may rise due to magnetic buoyancy to produce sunspots. In the flux transport dynamo model, the decay of tilted bipolar sunspot pairs gives rise to the poloidal field by the Babcock-Leighton mechanism. In this type of model, the meridional circulation of the Sun, which is poleward at the solar surface and equatorward at the bottom of the convection zone, plays a crucial role in the transport of magnetic fluxes. We finally point out that various stochastic fluctuations associated with the dynamo process may play a key role in producing the irregularities of the sunspot cycle.