THE EVERSHED EFFECT IN SUNSPOTS AS A SIPHON FLOW ALONG A MAGNETIC-FLUX TUBE

The Evershed effect(1)-a wavelength shift and profile asymmetry in the spectral lines observed from the outer regions of sunspots (the penum bra)-has been interpreted as a radial outflow of gas from the sunspot, but the dynamics of the now have not been fully understood(2). Althou gh the Evershed effect seems to stop abruptly at the outer edge of the penumbra, the outflow itself must continue, though tracing its path h as proved difficult. Theoretical(3,4) and observational(5-7) studies h ave suggested that much of the continuing now may follow magnetic held lines that go below the visible surface of the Sun at or just beyond the edge of the penumbra, and recent observations have now confirmed t his picture(8). Here we show, using theoretical calculations based on a more realistic model, that the flow acts like a siphon(3,9-12) which is driven along a magnetic nux tube by the pressure drop between the endpoints of the tube.