Abstract
The penumbra of a sunspot is a fascinating phenomenon featuring complex velocity and magnetic fields. It challenges both our understanding of radiative magneto-convection and our means to measure and derive the actual geometry of the magnetic and velocity fields. In this contribution we attempt to summarize the present state-of-the-art from an observational and a theoretical perspective.
We describe spectro-polarimetric measurements which reveal that the penumbra is inhomogeneous, changing the modulus and the direction of the velocity, and the strength and the inclination of the magnetic field with depth, i.e., along the line-of-sight, and on spatial scales below 0.5 arcsec. Yet, many details of the small-scale geometry of the fields are still unclear such that the small scale inhomogeneities await a consistent explanation.
A simple model which relies on magnetic flux tubes evolving in a penumbral “background” reproduces some properties of sunspot inhomogeneities, like its filamentation, its strong (Evershed-) outflows, and its uncombed geometry, but it encounters some problems in explaining the penumbral heat transport. Another model approach, which can explain the heat transport and long bright filaments, but fails to explain the Evershed flow, relies on elongated convective cells, either field-free as in the gappy penumbra or filled with horizontal magnetic field as in Danielson’s convective rolls. Such simplified models fail to give a consistent picture of all observational aspects, and it is clear that we need a more sophisticated description of the penumbra, that must result from simulations of radiative magneto-convection in inclined magnetic fields. First results of such simulations are discussed. The understanding of the small-scales will then be the key to understand the global structure and the large-scale stability of sunspots.
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