The Dynamics of the Excitation of Solar Oscillations

Abstract

We investigate seismic events, bursts of seismic waves that are generated locally just below the solar surface and that we detect traveling up through the photosphere. We identify a few thousand seismic events by their traveling wave character and find that they are associated with continuum darkening and downflow and have an extent of on average about 10-15 minutes and 1 Mm. Their birth rate is about 8 × 10-16 m-2 s -1. The observed upwardly traveling seismic flux in the average event (as derived from velocities in the p-mode region of k-ω space) is followed after about 3 minutes by some reflected downward flux. Only a small fraction of the energy generated in the hypocenter of the event below the surface travels straight up for us to see. The bulk of the generated energy is directed or reflected downward, and is eventually transformed into p-modes. The seismic events at the surface contain about 1.5 × 1019 J of seismic energy each, which corresponds to an average flux level of about 8.5 kW m-2 over the whole surface. The total energy flow is likely more than an order of magnitude greater, and is then in the same ballpark as the estimate of Libbrecht for the power required to sustain the p-mode spectrum. We find a roughly linear relation between the peak seismic flux and the peak downward convective velocity associated with each seismic event, which does not fit the highly nonlinear relations found theoretically by Lighthill and Goldreich & Kumar for stochastic excitation by turbulent convection, but does fit the monopole source deduced by Nigam & Kosovichev from a study of the p-mode spectrum.