EVIDENCE FOR A DOWNWARD MASS FLUX IN THE PENUMBRAL REGION OF A SUNSPOT

Sunspots were the first extraterrestrial phenomenon found to harbour m agnetic fields(1,2). But the physical nature of sunspots and their rel ationship to the Sun's global magnetic field are still poorly understo od(3). Perhaps the largest uncertainty is related to the outermost reg ion of sunspots (the penumbra) and, in particular, the nature of the s o-called Evershed(4) flow-a stream of material emanating radially from sunspots at velocities of up to similar to 6 km s(-1) (ref. 5), befor e vanishing abruptly at the outer penumbral edges. Here we make use of a recently developed optical tomographic technique(6) to obtain a thr ee-dimensional model of the magnetic held and mass now in the vicinity of a sunspot. We find that some of the magnetic field lines, together with a significant part of the Evershed mass nux, now back towards th e Sun in the deepest atmospheric layers at the outer edge of the sunsp ot and its surroundings. This observation should provide an important clue to our understanding of the appearance, stability and decay of su nspots, the most conspicuous tracers of the solar activity cycle.