Changes in the Subsurface Stratification of the Sun with the 11-Year Activity Cycle

We report on changes in the Sun's subsurface stratification inferred from helioseismology data. Using data from the Solar and Heliospheric Observatory (SOHO) Michelson Doppler Imager (MDI) for the last 9 years and, more precisely, the temporal variation of f-mode frequencies, we have computed the variation in the radius of subsurface layers of the Sun by applying helioseismic inversions. We have found a variability of the ``helioseismic'' radius in antiphase with the solar activity, with the strongest variations of the stratification being just below the surface, around 0.995 R_solar. In addition, the radius of the deeper layers of the Sun, between 0.975 and 0.99 R_solar, changes in phase with the 11-year cycle.