Observations of solar cycle variations in solar p-mode frequencies and splittings
We discuss here two sets of helioseismology data acquired at Big Bear Solar Observatory during the summers of 1986 and 1988. Each data set consists of roughly 60,000 fulldisk Doppler images of the sun, accumulated over a four-month time span. These data clearly show that solar p-mode frequencies change with time, and that the measured frequency shifts Δv = v₈₈ - v₈₆ depend strongly on frequency and only weakly on ℓ for 5 ≤ ℓ ≤ 60. The frequency dependence is well described by Δv ∞ M⁻¹(v), where M(v) is the mode mass for low-ℓ modes. Such a frequency dependence is expected if the effective sound speed perturbation is located predominantly near the solar surface. It should be possible to invert the frequency shift measurements to determine some aspects of the structure of solar activity as a function of depth. The data also show that the even-index splitting coefficients depend strongly on frequency, again being well described by α_(2j) (v) ∞ M⁻¹(v). This functional form is expected if the sound speed perturbation responsible for Δv is localized in solar latitude. Latitude inversions of the time-dependent splitting and Δv measurements show that the perturbation is strongest in the active latitudes, but includes a weak polar component.