Structural changes to the Sun through the solar cycle

Measurements of both solar irradiance and 5-min oscillation frequencies indicate that the structure of the Sun changes with the solar cycle. We report calculations addressing how global changes in the stratification of a star are induced by an internal thermal disturbance, and how this in turn affects the characteristic pulsation frequencies. The results indicate that deeply seated perturbations of the stratification invariably lead to relatively large radius changes over the cycle, contrary to observation. Moreover, the structural changes resulting from such perturbations predict oscillation frequency changes that do not reproduce the observed dependence on mode frequency. Therefore a magnetic layer at the base of the solar convection zone, for example, cannot be directly responsible for the dominant contribution to the observed structural changes. Disturbances near the surface of the Sun, on the other hand, lead to pulsation frequency changes which do reproduce observations. However, the luminosity is then predicted to decrease from solar minimum to maximum, by an amount that is about 45 times the increase observed. Thus purely thermal changes in the superficial layers of the Sun also fail to account for the observed structural variations.