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We analyze in detail the penumbral structure found in a recent radiative MHDsimulation. Near tau=1, the simulation produces penumbral fine structureconsistent with the observationally inferred interlocking comb structure. Fastoutflows exceeding 8 km/s are present along almost horizontal stretches of themagnetic field; in the outer half of the penumbra, we see opposite polarityflux indicating flux returning beneath the surface. The bulk of the penumbralbrightness is maintained by small-scale motions turning over on scales shorterthan the length of a typical penumbral filament. The resulting vertical rmsvelocity at tau=1 is about half of that found in the quiet Sun. Radial outflowsin the sunspot penumbra have two components. In the uppermost few 100 km, fastoutflows are driven primarily through the horizontal component of the Lorentzforce, which is confined to narrow boundary layers beneath tau=1, while thecontribution from horizontal pressure gradients is reduced in comparison togranulation as a consequence of anisotropy. The resulting Evershed flow reachesits peak velocity near tau=1 and falls off rapidly with height. Outflowspresent in deeper layers result primarily from a preferred ring-like alignmentof convection cells surrounding the sunspot. These flows reach amplitudes ofabout 50% of the convective rms velocity rather independent of depth. Apreference for the outflow results from a combination of Lorentz force andpressure driving. While the Evershed flow dominates by velocity amplitude, mostof the mass flux is present in deeper layers and likely related to alarge-scale moat flow.

Penumbral fine structure and driving mechanisms of large-scale flows in simulated sunspots