  The effects of radiative energy exchange on the convective instability of a
weak field magnetic structure, which lead to a prediction and a physical
explanation of the magnetic flux dependent field strength, are examined in
detail using a real model stratification for the photospheric and convection
zone structure of the Sun. Adopting the generalised Eddington approximation for
the radiative transfer, which is valid both in the optically thick and the thin
limits, we model the lateral radiative energy exchange by the tube with the
external medium with a self-consistent inclusion of vertical radiative losses.
