Explanation of the sea-serpent magnetic structure of sunspot penumbrae

TL;DR

This paper presents a 3D radiative MHD model explaining the sea-serpent magnetic structure in sunspot penumbrae, reproducing observed bipolar patches and wave-like magnetic field behavior through magnetoconvection and convective waves.

Contribution

It introduces the first 3D numerical model that reproduces downward magnetic fields and sea-serpent structures in sunspot penumbrae, linking them to magnetoconvection.

Findings

Reproduces observed bipolar magnetic patches in simulations.

Shows correlation between magnetic patches and Evershed downflows.

Demonstrates magnetoconvection as the cause of sea-serpent structures.

Abstract

Recent spectro-polarimetric observations of a sunspot showed the formation of bipolar magnetic patches in the mid penumbra and their propagation toward the outer penumbral boundary. The observations were interpreted as being caused by sea-serpent magnetic fields near the solar surface (Sainz Dalda & Bellot Rubio 2008). In this Letter, we develop a 3D radiative MHD numerical model to explain the sea-serpent structure and the wave-like behavior of the penumbral magnetic field lines. The simulations reproduce the observed behavior, suggesting that the sea-serpent phenomenon is a consequence of magnetoconvection in a strongly inclined magnetic field. It involves several physical processes: filamentary structurization, high-speed overturning convective motions in strong, almost horizontal magnetic fields with partially frozen field lines, and traveling convective waves. The results demonstrate a correlation of the bipolar magnetic patches with high-speed Evershed downflows in the penumbra. This is the first time that a 3D numerical model of the penumbra results in downward directed magnetic fields, an essential ingredient of sunspot penumbrae that has eluded explanation until now.