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 velocity and pressure oscillations of medium angular degree with that obtained from the Michelson Doppler Imager instrument on board the Solar and Heliospheric Observatory, we find that a composite source consisting of a monopole, which corresponds to mass or entropy fluctuations, and a quadrupole, which consists of the Reynolds stress, excites these oscillations. The dominant source 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 found to be correlated with the pressure perturbation. The sign of the correlation is found to be negative, which suggests that there is photospheric darkening prior to the occurrence of the localized acoustic event, in agreement with the previous finding of P. R. Goode and coworkers.