What dynamic changes in the sun drive the evolution of oscillation frequencies through the activity cycle?

The frequencies of solar oscillations (f- and p-modes) evolve through the solar cycle. The changes are driven by some combination of changes in the magnetic field, thermal structure and velocity field. It is unclear what is the precise combination of the three. One way or another, this thorny issue rests on an understanding of the response of the solar structure to increased magnetic field, but this is complicated. As well, we do not understand the origin of the sun's irradiance increase with increasing magnetic activity. Until recently, it seemed that an unphysically large magnetic field change was required to account for the frequency evolution during the cycle. However, the problem seems to have been solved (Dziembowski, Goode & Schou 2001). Specifically, a small-scale magnetic field was considered assuming uncorrelated field components - allowing the vertical component to be statistically different from the two horizontal ones. It turns out that a purely radial random field is the most economical, as well as being more physically sensible for other reasons. Furthermore, the solution might have a direct bearing on the origin of the irradiance variation. We discuss recent results and the present state of our knowledge.