Changes in convective properties over the solar cycle: effect on p-mode damping rates

Measurements of both solar irradiance and p-mode oscillation frequencies in dicate that the structure of the Sun changes with the solar cycle. Balmfort h, Gough & Merryfield investigated the effect of symmetrical thermal distur bances on the solar structure and the resulting pulsation frequency changes . They concluded that thermal perturbations alone cannot account for the va riations in both irradiance and p-mode frequencies, and that the presence o f a magnetic field affecting acoustical propagation is the most likely expl anation of the frequency change, in the manner suggested earlier by Gough & Thompson and by Goldreich et al. Numerical simulations of Boussinesq conve ction in a magnetic field have shown that at high Rayleigh number the magne tic field can modify the preferred horizontal length scale of the convectiv e flow. Here, we investigate the effect of changing the horizontal length scale of convective eddies on the linewidths of the acoustic resonant mode peaks obs erved in helioseismic power spectra. The turbulent fluxes in these model co mputations are obtained from a time-dependent, non-local generalization of the mixing-length formalism. The modelled variations are compared with p-mo de linewidth changes revealed by the analysis of helioseismic data collecte d by the Birmingham Solar-Oscillations Network (BiSON); these low-degree (l ow-l) observations cover the complete falling phase of solar activity cycle 22. The results are also discussed in the light of observations of solar-c ycle variations of the horizontal size of granules and with results from 2D simulations by Steffen of convective granules.