The Generation and Propagation of Waves in a Compressible Atmosphere.

Abstract

The equations governing the aerodynamic generation and the propagation of waves in a compressible atmosphere are exhibited. The fluctuating terms which are the turbulent sources for aerodynamic noise are approximated by an externally applied, time-harmonic, point force. Lighthill's results for the asymptotic radiation field in an anisotropic medium are then applied to an isothermal atmosphere. In this way, the surfaces of constant phase, group velocity, and intensity of the far field are computed. For finite frequencies above the critical frequency for vertical propagation, a monopole component is produced in the field by gravitational effects. The propagation problem is also studied for arbitrary temperature profiles, and it is found that in certain regions in the solar atmosphere there exist finite bands of non-propagating frequencies It is suggested that the oscillations in the solar atmosphere result from forced excitations of these non-propagating frequencies and that these are excited by turbulence arising from shear instability and penetrative convection