Seismic Observation of Solar Tachocline

Helioseismology has established the existence of a layer of strong gradients of angular velocity at the base of the solar convection zone. This layer (`tachocline') separates the convection zone exhibiting a strong latitudinal differential rotation from the radiative interior that rotates almost rigidly. Turbulence generated in the tachocline is likely to mix material in the upper radiative zone resulting in the observed deecit of Li and Be. However, the theoretical estimates of the precise location and the thickness of the tachocline depends on the details of turbulent energy and momentum transport, and are uncertain (Spiegel & Zahn, 1992). The tachocline layer is also the most likely place for the solar dynamo. Using rota-tional splitting coeecients of the p-mode frequencies, obtained during 1986-90 at the BBSO, we have found that the thickness of the transitional layer is about 0:090:04 solar radii, and that most of the transition occurs beneath the adiabatically stratiied part of the convection zone, as suggested by the dynamo theories of the 22-year solar cycle.