On Solar Frequency Changes
  The structure of the surface layers of the Sun is changed by magnetic activity which, in turn, changes the eigenfrequencies of the acoustic modes. These frequency shifts have been observed both in low- and high-degree data, and are found to be correlated with the solar cycle. Time-distance helioseismology has shown that waves travelling through sunspots suffer time delays. Also it is known that sunspots induce phase shifts between inwardly and outwardly propagating waves.   In the present work we show how these phase shifts are related to the observed time delays by treating the scattering of acoustic waves off sunspots as a diffraction problem.   These time delays are then related to the frequency shifts of the modes. From this relation we obtain a lower limit for the contribution of sunspots to the observed frequency variations with the solar cycle. Moreover, we predict the frequency dependence of the time delays suffered by rays travelling through a sunspot. 
M.Cunha, M.Bruggen, D.O.Gough