Properties of the acoustic source inferred from nonadiabatic oscillation spectra

Severino et al. (2001) showed that power and cross spectra of medium-l p-modes in velocity and intensity can be successfully fitted by splitting the noise in correlated, coherent, and uncoherent components. We show that if the pure oscillation is modelled by a numerical solution of the nonadiabatic boundary value problem which accounts for the radiative energy flux, the model of Severino et al. (2001) can be fitted to the observations without introducing ad hoc phase differences between the oscillations. From these fits, the phases and amplitudes of the correlated noise components can be estimated. The parameters provide information about the p-mode excitation process. Although the source depth can not be uniquely determined from the shape of the power and cross spectra, we show that the fraction of correlated noise is generally low (≤ 5%), and consistently higher in intensity than in velocity. The phase of the correlated intensity signal is such that the reduced intensity is ≈20° ahead of the downward mode velocity. This is qualitatively consistent with the idea of mode excitation in isolated rapid down-drafts in intergranular lanes. This idea is supported by simulations of the upper convection zone from Stein & Nordlund (2001).