Source of solar acoustic modes

Solar acoustic modes are found to be excited in a thin superadiabatic layer of turbulent convection (about 75 +/- 50 km below the photosphere) beneath the Sun's surface. Comparing the theoretical power spectra of both velocit y and pressure oscillations of medium angular degree with that obtained fro m the Michelson Doppler Imager instrument on board the Solar and Heliospher ic Observatory, we find that a composite source consisting of a monopole, w hich corresponds to mass or entropy fluctuations, and a quadrupole, which c onsists of the Reynolds stress, excites these oscillations. The dominant so urce is of a monopole type since it provides the best match to the observed velocity and intensity oscillation power spectra. For the above source to match the observed asymmetry in intensity, a part of the background is foun d to be correlated with the pressure perturbation. The sign of the correlat ion is found to be negative, which suggests that there is photospheric dark ening prior to the occurrence of the localized acoustic event, in agreement with the previous finding of P. R. Goode and coworkers.