  The rise of flux-tubes with intense magnetic fields from the base of the
convection zone to the solar surface has been substantiated as a probable
mechanism for sunspot formation. The origin of flux-tubes of sufficient
strength (~10^5 G) is however uncertain. This paper considers the instability
of a large-scale toroidal magnetic field caused by the magnetic suppression of
convective heat transport as a candidate for the flux tube forming mechanism.
The consideration employs the analytical dependence of the eddy diffusion on
the magnetic field supplied by mean-field magnetohydrodynamics. The instability
tends to produce regions of increased field strength with spatial scales of an
order of 100 Mm at the base of the convection zone. Characteristic growth times
of the instability are short compared to the 11-year cycle. The threshold field
strength for the onset of the instability increases from several hundred Gauss
in the vicinity of the equator to some kilo-Gauss at middle latitudes. Growth
rates of unstable disturbances decrease with latitude. These latitudinal trends
can be the reason for the observed confinement of sunspot activity to a
near-equatorial belt.
