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Recent spectro-polarimetric observations of a sunspot showed the formation ofbipolar magnetic patches in the mid penumbra and their propagation toward theouter penumbral boundary. The observations were interpreted as being caused bysea-serpent magnetic fields near the solar surface (Sainz Dalda & Bellot Rubio2008). In this Letter, we develop a 3D radiative MHD numerical model to explainthe sea-serpent structure and the wave-like behavior of the penumbral magneticfield lines. The simulations reproduce the observed behavior, suggesting thatthe sea-serpent phenomenon is a consequence of magnetoconvection in a stronglyinclined magnetic field. It involves several physical processes: filamentarystructurization, high-speed overturning convective motions in strong, almosthorizontal magnetic fields with partially frozen field lines, and travelingconvective waves. The results demonstrate a correlation of the bipolar magneticpatches with high-speed Evershed downflows in the penumbra. This is the firsttime that a 3D numerical model of the penumbra results in downward directedmagnetic fields, an essential ingredient of sunspot penumbrae that has eludedexplanation until now.

Explanation of the sea-serpent magnetic structure of sunspot penumbrae