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Penumbral Structure and Outflows in Simulated Sunspots

Sunspots Before Your Eyes Sunspots have a dark central region, the umbra, surrounded by a region of lighter radial filaments, the penumbra, along which there are outward horizontal mass flows that are still not fully understood despite their discovery 100 years ago. Now Rempel et al. (p. 171, published online 18 June; see the Perspective by Scharmer) present comprehensive numerical simulations of a pair of sunspots that show the development of an outer penumbra with systematic radial outflows along channels of nearly horizontal magnetic field in regions where the average field inclination is greater than 45 degrees. The outflows result from rising hot plasma that turns over and is guided outward by the strong and inclined magnetic field. The simulations reproduce observed properties throughout the entire penumbra and show that the penumbral structure and outflows in sunspots can be understood in terms of convective flow in a magnetic field with varying inclination. Simulations of sunspots show that their structure and outflows can be understood in terms of convection in a magnetic field. Sunspots are concentrations of magnetic field on the visible solar surface that strongly affect the convective energy transport in their interior and surroundings. The filamentary outer regions (penumbrae) of sunspots show systematic radial outward flows along channels of nearly horizontal magnetic field. These flows were discovered 100 years ago and are present in all fully developed sunspots. By using a comprehensive numerical simulation of a sunspot pair, we show that penumbral structures with such outflows form when the average magnetic field inclination to the vertical exceeds about 45 degrees. The systematic outflows are a component of the convective flows that provide the upward energy transport and result from anisotropy introduced by the presence of the inclined magnetic field.