Resonant coupling of ionization waves and acoustic gravity waves in the presence of a magnetic field

Resonant coupling between acoustic gravity waves in the atmosphere and ionization acoustic waves is possible for the same frequency for two different wave directions when a magnetic field is present. The particular frequency depends on the dip angle and the temperature and density of the atmosphere. The two wave number vectors for this critical coupling have equal components along the magnetic field line, and these components also depend on those quantities. Geometrical sketches in conjunction with analytical formulation are used to demonstrate the basis for the two resonant directions of propagation. The variation of resonance frequency and the variation of each of the wave vectors with dip angle are also explained in this fashion. The effects on coupling when the direction of propagation is out of the magnetic meridian are demonstrated by numerical calculations. It is also shown that coupling to acoustic branch waves in the atmosphere may occur. Mostly downward propagation is the rule, regardless of the branch of the acoustic gravity waves. It appears from numerical calculations that resonance may not be as localized in altitude as would be expected from the nature of its dependence on altitude. Resonancelike response is possible at a fixed frequency for an altitude range as large as 100 km. Further, peaked response of the ionization occurs for a range of directions exceeding 100 at altitudes of the order of 250 km. The angular range of directions is far greater at higher altitudes. Strong coupling is evident as well over a wide band of frequencies at a given altitude, the ratio of the highest to the lowest frequencies exceeding 3:1 with the resonance frequency being well inside the band. These findings strongly suggest that traveling ionispheric disturbances are connected with the coupling phenomenon, although experimental verification is essential