R. Howe, ^1* J. Christensen-Dalsgaard, ² F. Hill, ¹ R. W. Komm, ¹ R. M. Larsen, ³ J. Schou, ³ M. J. Thompson, ⁴ J. Toomre ⁵

We have detected changes in the rotation of the sun near the base of its convective envelope, including a prominent variation with a period of 1.3 years at low latitudes. Such helioseismic probing of the deep solar interior has been enabled by nearly continuous observation of its oscillation modes with two complementary experiments. Inversion of the global-mode frequency splittings reveals that the largest temporal changes in the angular velocity  are of the order of 6 nanohertz and occur above and below the tachocline that separates the sun's differentially rotating convection zone (outer 30% by radius) from the nearly uniformly rotating deeper radiative interior beneath. Such changes are most pronounced near the equator and at high latitudes and are a substantial fraction of the average 30-nanohertz difference in  with radius across the tachocline at the equator. The results indicate variations of rotation close to the presumed site of the solar dynamo, which may generate the 22-year cycles of magnetic activity.

¹ National Solar Observatory, National Optical Astronomy Observatories, Post Office Box 26732, Tucson, AZ 85726-6732, USA.
² Theoretical Astrophysics Center, Danish National Research Foundation, and Institute of Physics and Astronomy, Aarhus University, DK-8000 Aarhus C, Denmark.
³ Hansen Experimental Physics Laboratory, HEPL Annex, Stanford University, Stanford, CA 94305-4085, USA.
⁴ Astronomy Unit, Queen Mary and Westfield College, University of London, Mile End Road, London E1 4NS, UK.
⁵ JILA and the Department of Astrophysical and Planetary Sciences, University of Colorado, Boulder, CO 80309-0440, USA.
^*   To whom correspondence should be addressed. E-mail: rhowe{at}noao.edu