modes. In fact, $\delta\nu_{n,l}$ increases with increasing $l$.

We can think of the frequency dependence as a power law where $\delta\nu_{n,l}\propto\nu_{n,l}^\alpha$. A perturbation from a layer strictly confined to the photosphere (but extending over less than one pressure scale height) is expected to result in an $\alpha=3$ relationship [@1980tsp..book.....C; @1990LNP...367..283G; @1990Natur.345..779L; @1991ApJ...370..752G]. If instead the perturbation extends beneath the surface, the frequency dependence will be weaker and $\alpha$ will get smaller [@1990LNP...367..283G]. @2001MNRAS.324..910C determined that $\alpha<3$, which implies that the significant contribution comes from a perturbation close to the surface, in the sub-photospheric layers. @2008AdSpR..41..861R observed that $\alpha$ is smaller for the lower-frequency modes, which suggests that the perturbation extends to greater depths the lower in frequency one goes. This is consistent with the results of @2005ApJ...625..548D (see Section \[section\[global intro\]\]).

In summary, the oscillations are responding to changes in the strength of the solar magnetic activity near the Sun’s surface. As modes below approximately $1800\,\rm\mu
Hz$ experience almost no solar cycle frequency shift it is reasonable to conclude that the origin of the perturbation is concentrated in a region above the upper turning points of these modes. Fig. \[figure\[upper turning points\]\] shows that the upper turning point of a 