the propagation speed and the location of the upper turning point. Indirect effects can both increase or decrease the frequencies of p modes.

To date the relative contributions from the direct and indirect effects remain uncertain. @2005ApJ...625..548D suggest that the magnetic fields are too weak in the near-surface layers for the direct effect to contribute significantly to the observed frequency shifts and that the indirect effects dominate the perturbations. However, @2005ApJ...625..548D also suggest that the direct effect may be more important for low-frequency modes deeper within the solar interior where the magnetic field is strong enough to produce a noticeable shift in frequency. These results are still controversial and appear to disagree with the results of @1986Natur.323..603R, who found that field strengths of the order of 500kG would be required at the base of the convection zone to explain the shift. investigated the effect of a magnetic field at the base of the convection zone and a more shallow magnetic field, at a depth of 50Mm, but found their direct influence on the frequencies of p modes was consistently smaller than the change in frequency that is observed. Changes in both the chromospheric magnetic field strength and temperature can explain 