Variations in the excitation and damping of low-l solar p modes over the solar activity cycle

We have searched helioseismic data collected by the Birmingham Solar Oscill ations Network (BiSON) for solar-cycle changes to those low-L p-mode parame ters that relate to the excitation and damping of the resonances. These dat a - collected between 1991 and 1997 - cover the complete declining phase of solar activity cycle 22 (up to and including the cycle 22/23 boundary). Ov er the range 2600 less than or equal to nu less than or equal to 3600 mu Hz , we uncover a mean 24 +/- 3 per cent increase in the frequency-domain line widths; a mean decrease of 46 +/- 5 per cent decrease in the mode heights, and a mean decrease of 22 +/- 3 per cent in the modal velocity powers. The rate at which energy is supplied to the modes remains constant, at the leve l of precision of the observations (measured change 0 +/- 4 per cent). We u se expressions derived from the equation of a damped harmonic oscillator to illustrate the diagnostic properties of the observables: these indicate th at both the signs and relative sizes of the extracted variations can arise from changes solely to the net damping; the net forcing of the modes need n ot change. The results possibly hint at the changes being maximal at frequencies near similar to 3100 mu Hz. They might therefore suggest an origin for the obser ved variations that is peaked in the superadiabatic layer of the convection zone, which couples most strongly to the eigenfunctions of modes at the ce ntre of the p-mode spectrum.