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Paper: The Puzzling Structure of a Sunspot
Volume: 354, Solar MHD Theory and Observations: A High Spatial Resolution Perspective
Page: 213
Authors: Weiss, N.O.
Abstract: Sunspots are characterized by the presence of a filamentary penumbra but it is only within the last few years that the fine structure of penumbral magnetic fields, and of the associated Evershed outflow, has been definitively established. High resolution observations show that bright filaments in the inner penumbra possess slender dark cores with fields and flows that are nearly horizontal, while the ambient fields are inclined at 40° to the vertical. In the outer penumbra the fields in bright and dark filaments differ in inclination by about 30° and recent observations confirm that the Evershed flow is along the almost horizontal fields in dark filaments. Moreover, these two families of field lines remain distinct. This intricate magnetic geometry poses major theoretical problems. How can such a structure be maintained and how does it originate? How do penumbral fields relate to the photospheric granulation outside the spot? What drives the Evershed flow within dark filaments? What form does convection take in the umbra, in bright filaments and in dark filaments? What causes the fine structure within bright filaments? What is the subsurface structure of a sunspot and how does it relate to outflows and inflows in the moat cell that surrounds it? Although a general theoretical picture is beginning to emerge, these questions can only be properly answered through detailed computational investigations, guided by further observations both from the ground and from space.
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