On the measurement precision of solar p-mode eigenfrequencies

We make use of 3456 d of observations of the low-l p-mode oscillations of the Sun in order to study the evolution over time of the measurement precision of the radial eigenfrequencies. These data were collected by the ground-based Birmingham Solar-Oscillations Network (BiSON) between 1991 January and 2000 June. When the power spectrum of the complete time series is fitted. the analysis yields frequency uncertainties that are close to those expected from the returned coherence times of the modes. The slightly elevated levels compared with the prediction appear to be consistent with a degradation of the signal-to-noise ratio in the spectrum that is the result of the influence of the window function of the observations (duty cycle 71 per cent). The fractional frequency precision reaches levels of a several parts in 10(6) for many of the modes. The corresponding errors reported from observations made by the GOLF instrument on board the ESA/NASA SOHO satellite, when extrapolated to the length of the BiSON data set, are shown to be (on average) about similar to25 per cent smaller than their BiSON counterparts owing to the uninterrupted nature of the data from which they were derived. An analysis of the BiSON data in contiguous segments of different lengths, T, demonstrates that the frequency uncertainties scale as T-1/2. This is to be expected in the regime where the coherence (life) times of the modes, tau(nl), are smaller than the observing time T (the 'oversampled' regime). We show that mode detections are only now beginning to encroach on the 'undersampled' regime (where T < τ(nl)).