Magnetic diffusion in stratified atmospheres

The predictions from mean field electrodynamics have been questioned becaus e of the strong feedback of small-scale magnetic structure on the velocity fields. In two dimensions, this nonlinear feedback results in a lengthening of the turbulent decay time. In three dimensions a-quenching is predicted. Previous studies assumed a homogeneous fluid. We will present recent resul ts from two-dimensional compressible MHD decay simulations in a highly stra tified atmosphere that more closely resembles the solar convection zone. Ou r results show that the field geometry has a strong influence on the decay rate : vertical fields remain fairly constant for a period of time and then rapidly decay on the turbulent timescale; horizontal fields decay at an in termediate rate with strong fields persisting near the top and bottom bound aries. The implication of our results for understanding solar active region decay is discussed.