Tuesday

We compare the depth of the last two solar minima as measured by the frequencies of helioseismic oscillations using data from the Global Oscillation Network Group (GONG). We find that the minimum following Cycle 23 is deeper than the minimum preceding Cycle 23 and that the difference is statistically significant, indicating that the global magnetic field was weaker in the minimum following Cycle 23. We find that the magnitude of the difference in frequency of the oscillations between the two minima is dependent on the frequency of the oscillation itself and that the functional form of this dependence implies a near-surface perturbation, restricted to the outer 0.5% of the solar interior by radius. This result demonstrates that helioseismic oscillations are a precise measure of small changes in the Sun’s global magnetic field. It also raises questions over the unperturbed helioseismic and asteroseismic frequencies that are inputs in solar and stellar models and the potential minima magnetic state of the Sun.