  A theoretical sunspot model is provided including magnetic suppression of the
diffusivities and also a strong stratification of density and temperature. Heat
diffusion alone with given magnetic field and zero mean flow only produces
(after a very long relaxation time) dark spots without any bright ring. Models
with full dynamics of both field and flow, however, provide rings and also the
observed correlation of ring temperature excess and the spot size. The rings
are formed as the result of heat transport by the resulting flow system and
increased thermal diffusivity due to reduced magnetic quenching around spots.
