field. Although the correlation between the even $a$ coefficients and global measures of the Sun’s magnetic field is good, it is not linear. However, the even splitting coefficients are linearly correlated with the corresponding Legendre polynomial decomposition of the surface indicating that the size of the observed frequency shift experienced by a particular mode is dependent on the latitudinal distribution of the surface magnetic flux. Furthermore, @2002ApJ...580.1172H showed that it is possible to use latitudinal inversion techniques to localize the frequency shifts in latitude and, in fact, reconstruct the evolution of the surface magnetic field. An up-to-date version of these inversions can be seen in Fig. \[figure\[shift\_butterfly\]\], which clearly resembles the familiar butterfly diagrams usually associated with the surface magnetic field.

![Frequency shift inversions using GONG data and modes with $40\le l <80$, and $9\le n \le 11$. The size of the shift is indicated by the colour and the black contours show the surface magnetic field at 5G intervals. Updated from @2002ApJ...580.1172H.[]{data-label="figure[shift_butterfly]"}](fshift.jpg "fig:"){width="60.00000%"}\

Evidence for structural changes due to magnetic fields deeper in the interior
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All that has been discussed above concerns the near-surface magnetic field. However, one of the main advantages of helioseismology is that it allows 