An 88-year-old mystery surrounding sunspots has finally been solved, thanks to the ingenious application of an optical tomographic imaging technique.

Sunspots are relatively cool regions of the solar disk where bundles of magnetic field lines break through the surface. A puzzling phenomenon, first noted by J. Evershed in 1909 (Mon. Not. R. Astron. Soc. 69, 454-457; 1909), concerns the apparent violation of one of the basic laws of physics — conservation of mass. Spectroscopic observations of sunspots show a flow of material streaming across the face of the spot, only to perform a mysterious vanishing act at its outer edge.

But new false-colour maps (right) of a typical sunspot, obtained by C. Westendorp Plaza et al. and published in this issue (Nature 389, 47-49; 1997), reveal the secret of these solar Bermuda Triangles. The images were constructed using an indirect tomographic method, which involves mathematical processing of optical polarization data. The results, at different depths in the solar atmosphere, are shown in the upper two panels. (The bottom panel is a conventional optical image of the spot.) Importantly, the technique yields information not only about the velocity of material within the vicinity of a sunspot, but also about the magnetic field (represented by arrows).

The new tomography implies that the field lines form closed loops in the deepest layers of the atmosphere, near to the solar surface. This is a crucial observation, because theory suggests that gas should be physically routed along these field lines, or ‘flux tubes’. And indeed, in the outermost regions of the spot, inflowing material is strongly correlated with inwardly directed magnetic flux (the blue-shaded regions). So the sink of material is at last revealed — at the edge of the spot, the low-lying magnetic flux tubes drag the gas back into the Sun, rendering the flow spectroscopically invisible.

Controversy has often surrounded the Evershed effect — for example, some earlier observations seemed to contradict it, in that they failed to indicate an abrupt termination of the flow at the spot edge. But now the reason is clear. The key point is that physical quantities (such as magnetic flux) vary with height within the solar atmosphere. Previous measurements were incomplete in that they could only sample one particular layer, as in the fabled story in which an elephant is described by blind people. But the 3D tomographic technique can probe these gradients, and measure the field and mass flow within the deepest atmospheric layers.

It is now clear from the tomography that, in the higher atmospheric layers, the magnetic field lines are open, rather than being closed loops, and extend beyond the visible limits of the spot. Therefore, at this height, the flow should not abruptly terminate at the spot edge, which explains the apparently anomalous earlier observations of these layers.

So in more ways than one, the Evershed flow could be said to be an open-and-shut case.

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