Response of the radial stratification at the base of the convection zone to the activity cycle

We have analyzed the differences in the oscillation eigenfrequencies measur ed over more than two years of GONG observations (December 1995-April 1998) and SOI/MDI observations (May 1996-July 1998). We have inverted these data to look for signatures of the solar activity cycle on the stratification. A one-dimensional (radial) structure inversion code was used to study tempo ral variations of the sound speed distribution at the boundary between the radiative and convective zones. Such variations could be an indication of t he presence of a toroidal magnetic field anchored in this region.