Radiative transfer effects and the dynamics of small-scale magnetic structures on the Sun

The dynamical consequences of radiative energy transport on the evolution o f gas confined to small-scale magnetic structures on the Sun are studied. C onvective collapse, which transforms weak-field structures into intense str uctures of field strengths in the 1-2 kG range on the photosphere, is stron gly in influenced by radiative heating from the surroundings and cooling du e to losses in the vertical direction. We first present analytic results in the quasi-adiabatic approximation to attempt a qualitative understanding o f the influence of radiative effects on the convective stability of Bur tub es. We demonstrate the destabilizing action of vertical radiative losses, t hat tend to enhance convective collapse and produce strong tubes at a relat ively smaller horizontal scale than those expected from calculations based solely on horizontal radiative energy transport. Our calculations clearly p oint to an asymmetry between upflow and downflow perturbations-only the lat ter are amplified in the presence of vertical radiative transport. Using a realistic model of the solar atmospheric structure and treating radiative t ransfer in the diffusion and Eddington approximations, we next perform nume rical stability analyses and produce size (flux)strength relations for sola r flux tubes. Our results provide a physical explanation for the observed f lux-dependent (equivalently size-dependent) field strengths of the solar sm all-scale magnetic structures in the form of weak intranetwork and strong n etwork components.