  The questions whether the Sun shrinks with the solar activity and what causes
this have been a subject of debate. Helioseismology provides means to measure
with high precision the radial displacement of subsurface layers, co-called
"seismic radius", through analysis of oscillation frequencies of surface
gravity (f) modes. Here, we present results of a new analysis of twenty one
years of helioseismology data from two space missions, Solar and Heliospheric
Observatory (SoHO) and Solar Dynamics Observatory (SDO), which allow us to
resolve previous uncertainties and compare variations of the seismic radius in
two solar cycles. After removing the f-mode frequency changes associated with
the surface activity we find that the mean seismic radius is reduced by 1-2 km
during the solar maxima, and that most significant variations of the solar
radius occur beneath the visible surface of the Sun at the depth of about 5 Mm,
where the radius is reduced by 5-8 km. These variations can be interpreted as
changes in the solar subsurface structure caused by predominately vertical ~10
kG magnetic field.
