Penumbral fine structure: Theoretical understanding

To understand the dynamic fine structure and the Evershed effect of the sunspot penumbra, we have carried out time-dependent simulations of a thin magnetic flux tube embedded in a static sunspot model (Schlichenmaier et al. 1998a,b). Here, we present new simulation results of the moving tube model which reveal flux tubes that, instead of lying horizontal in the penumbral photosphere, develop waves (photospheric serpent) that reproduce not only the inward migration of penumbral grains, but also their observed outward migration, and which exhibit downflow arches in the outer penumbra. During its evolution the flux tube exhibits quasi-steady states. These states are compared to time-independent solutions of siphon flow models, which have been used to explain the Evershed flow (Meyer & Schmidt 1968, Degenhardt 1991, Montesinos & Thomas 1997).