ON SOUND GENERATION BY TURBULENT CONVECTION - A NEW LOOK AT OLD RESULTS

We have revisited the problem of acoustic wave generation by turbulent convection in stellar atmospheres. The theory of aerodynamically gene rated sound, originally developed by Lighthill and later modified by S tein to include the effects of stratification, has been used to estima te the acoustic wave energy flux generated in solar and stellar convec tion zones. In this paper we correct the earlier computations by incor porating an improved description of the spatial and temporal spectrum of the turbulent convection. We show the dependence of the resulting w ave fluxes on the nature of the turbulence, and compute the wave energ y spectra and wave energy fluxes generated in the Sun on the basis of a mixing-length model of the solar convection zone. In contrast to the previous results, we show that the acoustic energy generation does no t depend very sensitively on the turbulent energy spectrum. However, t ypical total acoustic fluxes of order F(A) = 5 x 10(7) ergs cm-2 s-1 w ith a peak of the acoustic frequency spectrum near omega = 100 mHz are found to be comparable to those previously calculated. The acoustic f lux turns out to be strongly dependent on the solar model, scaling wit h the mixing-length parameter alpha as alpha3.8. The computed fluxes m ost likely constitute a lower limit on the acoustic energy produced in the solar convection zone if recent convection simulations suggesting the presence of shocks near the upper layers of the convection zone a pply to the Sun.