GLOBAL oscillations of the Sun1 have been used to test solar models2, but modelling the oscillation frequencies to their measured accuracies of a few microhertz has proved difficult, mostly owing to ignorance of the structure of the Sun's outer layers3. The frequency separation between closely spaced modes in the acoustic spectrum is expected to depend more on core properties4, however, and thus to provide constraints on models of the solar core. Our observations combine data from a global network of observing stations, which reduces the masking effect of daily sidebands in the spectral analysis. Here we present precision measurements of fine structure and its variation with frequency. Our results agree with standard solar models5-7, and seem to remove the need for significant mixing8,9 or weakly interacting massive particles (WIMPS)10,11 in the core, both of which have been advanced to explain the low measured flux of solar neutrinos12,13. This suggests that the solar neutrino problem must be resolved within neutrino physics, not solar physics; neutrino oscillations and a finite neutrino mass form a possible explanation.