Multi-Ridge Fitting for Ring-Diagram Helioseismology

Inferences of sub-surface flow velocities using local domain ring-diagram helioseismology depend on measuring the frequency splittings of oscillation modes seen in acoustic power spectra. Current methods for making these measurements utilize maximum-likelihood fitting techniques to match a model of modal power to the spectra. The model typically describes a single oscillation mode, and each mode in a given power spectrum is fit independently. We present a new method that produces measurements with greater reliability and accuracy by fitting multiple modes simultaneously. We demonstrate how this method permits measurements of sub-surface flows deeper into the Sun while providing higher uniformity in data coverage and velocity response closer to the limb of the solar disk. While the previous fitting method performs better for some measurements of low-phase-speed modes, we find this new method to be particularly useful for high phase-speed modes and small spatial areas.