Simulations of erupting flux tubes in the thin flux tube approximation show that essential properties of sunspots can only be explained if the initial field strength of the flux tube at the base of the convection zone is about 10 T. Such strong magnetic field can only be stored below the solar convection zone in a subadiabatic stratification. We consider mechanical equilibria in form of magnetic flux tubes and magnetic sheets and discuss the influence of radiative and convective energy transport on these configurations. In the case of magnetic flux tubes, radiative inflow of heat leads to enhanced buoyancy which causes the flux tube to move upwards and leave the storage region. In the case of magnetic sheets, the compensation of the poleward directed magnetic tension force requires a deviation of the temperature from the hydrostatic background stratification. Convective energy transport disturbs the equilibrium and leads to thermal circulations.