the magnetic field that is observed at the solar surface (and beyond into the solar atmosphere).

Comparison between frequency shifts and the surface magnetic field {#section[surface comp]}
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Numerous comparisons have been made between the p-mode frequency shifts and various proxies of the Sun’s surface magnetic field , including the 10.7cm and the International Sunspot Number[^3] (ISN). @2007ApJ...659.1749C compared six different proxies with the low-$l$ mode frequency shifts and demonstrated that better correlations are observed with proxies that measure both the strong and the weak components of the Sun’s magnetic field, such as the Mg II H and K core-to-wing data, the 10.7 cm radio flux, and the He I equivalent width data (as opposed the the ISN and the Kitt Peak Magnetic Index, which mainly sample the strong magnetic flux). The weak-component of the solar magnetic flux is distributed over a wider range of latitudes than the strong component and so a possible explanation of these results is in terms of the latitudinal distribution of the modes used in this study (i.e. low-$l$ modes).

![Comparison of frequency shifts with surface proxies of the Sun’s magnetic field. The left-hand panel shows a comparison with the 10.7cm flux, while the right-hand 