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A subsurface ow of material from the Suns equator to its poles
P. M. Giles*, T. L. Duvall Jr, P. H. Scherrer & R. S. Bogart
* Stanford University, Department of Applied Physics, Stanford, California 94305-4085, USA Laboratory for Astronomy and Solar Physics, NASA/Goddard Space Flight Center, Greenbelt, Maryland 20771, USA W. W. Hansen Experimental Physics Laboratory, Stanford University, Stanford, California 94305-4085, USA
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Gas on the Suns surface has been observed14 to ow away from
the equator towards both poles. If the same ow persists to great depths, it could play an important dynamical role in the eleven-year sunspot cycle, by carrying the magnetic remnants of the sunspots to high latitudes5. An even deeper counterow, which would be required to maintain mass balance, could explain why new sunspots form at lower latitudes as the cycle progresses6.
These deep ows would also redistribute angular momentum within the Sun, and therefore help to maintain the faster rotation of the equator relative to the poles7. Here we report the detection, using helioseismic tomography, of the longitude-averaged subsur-face ow in the outer 4% of the Sun. We nd that the subsurface ow is approximately constant in this depth range, and that the speed is similar to that seen on the surface. This demonstrates that the surface ow penetrates deeply, so that it is likely to be an important factor in solar dynamics.
An axisymmetric ow in meridian planes is known in astrophysics as a meridional circulation. This type of ow in stars has a long history, having been rst proposed in 1925 by Eddington8. In regions of stars that lack convective motions, the importance of these ows for stellar evolution is that they can mix different layers of the stellar material and hence destroy composition gradients. Of course these ows have not been observed directly in distant stars, as they have small magnitudes and are difcult to isolate without some resolution of the stellar disk.
For the Sun, the ow speed is small compared with the random motions at the surface (1 km s1) and with the solar rotation (2 km s1) and so has been difcult to observe. From the surface measurements, there is rough consensus that the ow is poleward in both hemispheres,...