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Published on May 31, 2011
Time-lapse movie of the sunspot acquired in violet light. Colourized for aesthetic reasons. The sequence corresponds to 53 minutes in real time. The penumbral filaments seem to move inwards, but the gas flow is actually mainly directed outwards, away from the dark umbra.
What physical processes shape and heat the penumbra? Is it convection of the kind that is working just below the ordinary, or "quiet" solar surface? There, the convection gives rise to the pattern called granulation where hot bright gas is rising and cold dark gas is sinking. If similar processes are important for the penumbra, there should be a systematic pattern of up-and-down motions in the gas. But such a pattern has not been seen — until now.
The downflows identified in the current study are statistically associated with darker regions and must be convective in nature. The conclusion is that the penumbra is powered, and to a large extent structured, by a convective flow from below. This result, which is supported by recent simulation results, should settle a long-standing scientific debate.
The convective downflows and upflows are associated with a strong (several km/s) horizontal outflow directed radially outwards from the sunspot centre. These horizontal flows constitute the famous and mysterious Evershed effect. The identification of the penumbral flows as being convective in origin finally explains this century-old mystery. The Evershed effect was discovered in 1909 by John Evershed at the Kodaikanal Observatory in India.
The results are published in Science Express. The authors are: Göran B. Scharmer, Vasco M.J. Henriques, Dan Kiselman, (all from the Institute for Solar Physics of the Royal Swedish Academy of Sciences and the Department of Astronomy at Stockholm University), and Jaime de la Cruz Rodríguez (University of Oslo).
The Swedish 1 m Solar Telescope is operated on La Palma by the Institute for Solar Physics of the Royal Swedish Academy of Sciences in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrof\'isica de Canarias.