The excitation and damping of solar oscillations

Abstract

Present theories suggest two classes of excitation processes which may be responsible for the observed amplitudes of solar p-mode oscillations—self-excitation of the modes via an overstability mechanism such as the K mechanism ², and stochastic excitation by turbulent convection^2–4. We now have data which stand in support of the latter mechanism. Linear overstability calculations are still ambiguous, because of the uncertainties involved in including turbulent viscous damping of the modes. Nevertheless no calculation predicts overstable f-modes, and these modes are observed on the Sun. Furthermore no nonlinear damping mechanism has yet been proposed which would limit the growth of overstable modes to their observed amplitudes. Assuming the modes are stably damped, the theory of mode excitation by convective turbulence now gives mode amplitudes that are in agreement with observations⁴. In this letter we use data from Big Bear Solar Observatory to compare theory and observation in detail. In particular, our data show that the energy per oscillation mode is nearly constant at low mode frequencies, and is approximately independent of degree at low degrees. The total energy in all the oscillation modes is estimated at ∼10³⁴erg.