Cyclic Changes of the Sun’s Seismic Radius

The questions asking whether the Sun shrinks with the solar activity and what causes this have been a subject of debate. Helioseismology provides a means to measure with high precision the radial displacement of subsurface layers, the so-called "seismic radius," through the analysis of oscillation frequencies of surface gravity (f) modes. Here, we present results of a new analysis of 21 years of helioseismology data from two space missions, the Solar and Heliospheric Observatory and the Solar Dynamics Observatory, 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 a depth of about 5 ± 2 Mm, where the radius is reduced by 5–8 km. These variations can be interpreted as changes in the solar subsurface structure caused by the predominately vertical ~10 kG magnetic field.