Small separations and phase shift differences of l=0, 1 p-modes

Aims. We investigate the diagnostic potential of l = 0, 1 p-modes and the origin of the periodicity in their small separations. Methods. We used theoretical analysis, phase- shifts, modelling, and data analysis. Results. The periodicity in the small separations between modes of l = 0, 1 is determined by the acoustic radius of the base of the outer convective envelope. The mean variation is determined primarily by the structure of the inner core. The separations are related to the inner phase shifts differences delta(1) - delta(0) which we show can be determined directly from the frequencies. The modulation period is shifted slightly by the frequency dependence of the phase shifts and the amplitudes. We present results using data from the BiSON, IRIS, and GOLF experiments, and a solar model, all of which give a modulation period of similar to 359 +/- 5 mu Hz corresponding to an acoustic radius similar to 1422 +/- 20 s.