at about $3000\,\rm\mu
Hz$ the change in frequency between solar maximum and minimum is about $0.4\,\rm\mu Hz$. By examining the changes in the observed p-mode frequencies throughout the solar cycle we can learn about solar-cycle-related processes that occur beneath the Sun’s surface. The 11 year cycle is seen clearly in Fig. \[figure\[BiSON shifts\]\], which shows the mean frequency shifts of the p modes observed by BiSON and the 10.7cm flux[^2] for comparison .

![Average frequency shifts of “Sun-as-a-star” modes with frequencies between 2.5 and $3.5\,\rm mHz$. The results were obtained from 365d BiSON time series that overlapped by 91.25d. Also plotted is a scaled and shifted version of the 10.7cm flux.[]{data-label="figure[BiSON shifts]"}](freq_shifts_vs_time_2500_3500_365d_flux_incl_old.jpg "fig:"){width="60.00000%"}\

What causes the observed frequency shifts? The magnetic fields can affect the modes in two ways: directly and indirectly. The direct effect occurs because the Lorentz force provides an additional restoring force resulting in an increase of frequency, and the appearance of new modes. Magnetic fields can influence the oscillations indirectly by affecting the physical properties of the cavities in which the modes are trapped and, as a result, the propagation of the acoustic waves within them. For example, changes in the thermal structure can affect both 