  We have analyzed changes in the acoustic oscillation eigenfrequencies
measured over the past 7 years by the GONG, MDI and LOWL instruments. The
observations span the period from 1994 to 2001 that corresponds to half a solar
cycle, from minimum to maximum solar activity.
  These data were inverted to look for a signature of the activity cycle on the
solar stratification. A one-dimensional structure inversion was carried out to
map the temporal variation of the radial distribution of the sound speed at the
boundary between the radiative and convective zones. Such variation could
indicate the presence of a toroidal magnetic field anchored in this region.
  We found no systematic variation with time of the stratification at the base
of the convection zone. However we can set an upper limit to any fractional
change of the sound speed at the level of $3 \times 10^{-5}$.
