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R. Howe (Nat. Sol. Obs.), J. Christensen-Dalsgaard (TAC/ Aarhus U.), F. Hill, R.W. Komm (Nat. Sol. Obs.), R.M. Larsen, J Schou (Stanford U.), M.J. Thompson (QMW, U. London), J. Toomre (U. Colorado)
The differential rotation of the Sun and its ability to generate large-scale magnetic fields through cyclic dynamo action appear to be intimately linked. It is now commonly thought that the global dynamo behavior responsible for the emergence of active regions is derived from strong organized toroidal magnetic fields generated by rotational shear in a thin region (the tachocline) at the base of the convection zone. The magnetic field could well have a feedback effect on the fluid motions in the region. We are thus motivated to use helioseismology to look for changes in rotation profiles near the tachocline as the Sun's magnetic cycle progresses.
This approach has become possible using frequency-splitting data for p- and f-mode oscillations derived over four years (from May 1995 to Sept 1999) of full-disk Doppler observations from the ground-based Global Oscillation Network Group (GONG) project and from the Michelson Doppler Imager (MDI) experiment aboard the SOHO spacecraft. Inversions using two different methods of the splittings from these two independent data sets reveal systematic variations of the rotation rate close to the base of the convection zone, with different behavior at low and high latitudes. Notable are variations of order 6 nHz in rotation rates near the equator, to be compared with the radial angular velocity contrast across the tachocline of about 30 nHz. These exhibit several nearly repetitive changes with a period of about 1.2-1.4 years and appear to be real changes in the deep convection zone and tachocline rotation rates that need to be followed as the solar cycle progresses.
The GONG project is managed by the National Solar Observatory, a Division of the National Optical Astronomy Observatories, which is operated by AURA, Inc. under a cooperative agreement with the National Science Foundation. SOHO is a joint project of ESA and NASA.