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The influence of ionization and conduction on the dispersion of acoustic gravity waves in the upper atmosphere has been investigated by means of an analysis of the corresponding dispersion relation, when the geomagnetic field is directed horizontally. It is shown that we have to distinguish two situations, i.e., acoustic gravity waves propagating perpendicular or nearly perpendicular to the Earth's magnetic field and acoustic gravity waves propagating oblique or parallel to the Earth's magnetic field. This last situation is the one considered in a paper by Hines and Hooke (1970). In the present paper both situations are described analytically; the transition between the two is studied numerically. A consequence of the incorporation of ionospheric conductivity is a damping of acoustic gravity waves with a damping rate lying between a characteristic damping rate that scales with the Pedersen conductivity and zero, depending on the frequency of the wave and the direction of propagation. This angular and frequency dependence of the damping rate may be experimentally verifiable.

A theoretical study is presented of multi-wave mixing dynamics in a single-mode semiconductor laser with monochromatic weak external injection. Three relevant regimes are overviewed, i.e. corresponding to locking. four-wave mixing and multi-wave mixing. Moreover, much emphasis is put on four-wave mixing. For this regime, several analytical expressions are presented, some of which are new. A detailed theoretical explanatioiL is given for the peculiar relaxation oscillation resonance behavior that was recently observed in experiments.