Evidence for a downward mass flux in the penumbral region of a sunspot

Sunspots were the first extraterrestrial phenomenon found to harbour magnetic fields(1,2). But the physical nature of sunspots and their relationship to the Sun's global magnetic field are still poorly understood(3). Perhaps the largest uncertainty is related to the outermost region of sunspots (the penumbra) and, in particular, the nature of the so-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), before vanishing abruptly at the outer penumbral edges. Here we make use of a recently developed optical tomographic technique(6) to obtain a three-dimensional model of the magnetic field 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 the Sun in the deepest atmospheric layers at the outer edge of the sunspot and its surroundings. This observation should provide an important clue to our understanding of the appearance, stability and decay of sunspots, the most conspicuous tracers of the solar activity cycle.