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

Abstract

We have searched helioseismic data collected by the Birmingham Solar Oscillations Network (BiSON) for solar-cycle changes to those low-ℓ p-mode parameters that relate to the excitation and damping of the resonances. These data — collected between 1991 and 1997 — cover the complete declining phase of solar activity cycle 22 (up to and including the cycle 22/23 boundary). Over the range 2600≤ν≤3600 μHz, we uncover a mean 24±3 per cent increase in the frequency-domain linewidths; 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 level of precision of the observations (measured change 0±4 per cent). We use expressions derived from the equation of a damped harmonic oscillator to illustrate the diagnostic properties of the observables: these indicate that 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 not change. The results possibly hint at the changes being maximal at frequencies near ∼3100 μHz. They might therefore suggest an origin for the observed variations that is peaked in the superadiabatic layer of the convection zone, which couples most strongly to the eigenfunctions of modes at the centre of the p-mode spectrum.