Cyclic variability of the seismic solar radius from SoHO/MDI and related physics

We report on the changes of the Sun's subsurface stratification inferred from helioseismic data. We have used SoHO/MDI f-mode frequencies and their temporal variation for the last 10 years to compute the variation of the 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_⊙. On the contrary, the deeper layers of the Sun, between 0.975 R_⊙ and 0.99 R_⊙ seem to change in phase with the 11-year cycle. These results imply a non-homogeneous variation of the subsurface layers with depth and time. We also found variations in the even-a coefficients both with the frequency and the cycle that imply the existence of asphericities in the subsurface layers. We also discuss the influence of the magnetic field, turbulent pressure and temperature on these variations. Finally, we will discuss of the physical processes which mainly act in this region and conclude on how we may progress on this transition region between solar interior and external part.