SOLAR PULSATIONAL STABILITY .1. PULSATION-MODE THERMODYNAMICS

It is not currently known what excites solar five-minute oscillations. Of the two most plausible possibilities, thermal overstability and stochastic excitation by turbulent convection, the single most important discriminating factor is the intrinsic stability of the pulsation modes. In view of this fact, we address the problem of the linear stability of model solar envelopes. We employ a time-dependent, non-local mixing-length prescription for convection, and the Eddington approximation to radiative transfer. The calculations reveal that low-degree acoustic modes are damped. Moreover, the theoretical damping rates compare well with measurements of solar oscillation line widths. Turbulent pressure fluctuations play a critical role in stabilizing the pulsations. Finally we relate our results to those of previous investigations.