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Session 74 - The Quiet & Active Sun.
Display session, Friday, January 09
Exhibit Hall,
A classic problem in solar magnetohydrodynamics has been to explain the Evershed effect, a wavelength shift and asymmetry of spectral line profiles which indicate a nearly horizontal, radial outflow of gas across a sunspot penumbra. Although the Evershed effect ceases rather abruptly at the outer edge of the penumbra, the flow itself must continue in some way, and tracing the course of this flow has proved difficult. The siphon-flow model, in which the Evershed flow is driven along an arched magnetic flux tube by a pressure drop between the two footpoints of the tube, predicts that much of this flow is directed along magnetic field lines that dive below the solar surface near the outer edge of the penumbra. Recent observations have suggested that this is indeed true, and now the very recent observations of Westendorp Plaza et al.\ (WP; 1997, Nature, 389, 47) have confirmed this picture. We present the results of our latest siphon-flow model of the Evershed flow, which allows us to follow flux tubes that cross the outer penumbral boundary into the surrounding field-free photosphere, as well as flux tubes that return to the surface within the penumbra. Both of these cases are observed by WP, and the results of our model are in good quantitative agreement with these observations.
