A reexamination of evanescent acoustic‐gravity waves: Special properties and aeronomical significance

Acoustic‐gravity wave relations indicate that when wave frequency and horizontal wave number approach the characteristic curve‐delineating gravity and acoustic solutions, the horizontal group velocity and Eckart's characteristic impedance become infinite and wave energy E vanishes. It is shown that this behavior is equivalent to assuming incorrectly that wave energy is the same function of vertical wave number for internal and evanescent waves. When the correct form of E for evanescent waves is used, the energy flow velocity U defined in terms of the energy flux F x = EU is bounded by the sound speed, impedance is bounded by values near unity, and E does not vanish. For waves near the Lamb‐wave solution, the dominant dynamical control of the vertically integrated airglow response is the horizontal divergence. The only significant long‐lived evanescent response following an excitation event should be near the Lamb curve, and here the horizontal divergence is largest for small‐scale high‐frequency waves. The transient response is strongest for those waves that disperse least: waves with long‐horizontal wavelengths and waves not too far from the Lamb and Brunt‐Vaisala (BV) curves. Wave properties are well behaved near the characteristic curve and solutions are linked at the common intersection of the Lamb and BV curves implying that waves should be able to transfer energy from one regime to the other in response to background variations. Nonisothermal calculations show that when lapse rates are large enough waves that are at least partially internal can exist at all frequencies.