sional or radiative relaxation rate is much smaller than the photoexcitation rate. The environmental consequences of plasma-chemistry perturbations are uncertain. Any mechanism which alters the electron density or the relevant temperatures of the various species (T, Th T,} will have some environmental ramifications. The manifestations of these phenomena, such as enhanced airglow and changes in hf communications, are not fully understood. Probably the worst consequence of any space-to-earth power beaming scheme would be depletion of the ozone layer which protects the earth’s surface from harmful uv radiation. Most of the ozone is found below an altitude of 50 km. Some depletion above that altitude would have little consequence; however, ozone does not absorb in the mid-IR wavelengths and no direct photodestruction scheme at any altitude was identified in this study. Secondary reaction channels (discussed below) may alter the Ot concentration, but only at high altitudes (>60 km) and only in a localized manner. Vibrational Photoexcitation. Sunshine and earthshine are the primary radiation sources for photoexcitation of upper-atmosphere molecules under undisturbed conditions. The earthshine appears to be the most important source of vibrational excitation for wavelengths >5 p.m (58). Among the molecular species of the upper atmosphere, the metal oxides are most susceptible to this source of excitation, since their band fundamentals tend to fall near the peak of the earthshine irradiance spectrum and collisional quenching of their vibrational states is slow. Because the CO laser spectrum consists of a number of discrete lines, absorption transitions would need to be coincident with the laser lines for significant photoexcitation to occur. While the detailed transition levels of the various metal oxides have not been examined in detail, the concentrations of metal oxides are so low and the laser-beam area is so small that the likelihood of serious ionospheric perturbation is insignificant. Furthermore, almost all of the metal oxides, such as A1O, FeO, etc., reradiate in the infrared and, hence, no visible airglow would be observed. Charged-Species Reactions. Most of the charged-species reactions of interest to the present discussion are possible only in the D-region. The D-region is the most chemically complex region of the ionosphere (59) and the exact nature of possible laser induced perturbation is difficult to predict without detailed kinetic modeling. The D-region consists of a large concentration of neutral species in which positive and negative ions are the principal charge carriers and complex ion-interchange and electron attachment and detachment reactions occur (60). Photoreactions in the D-region which may be induced by single or multiphoton processes involving an intense photon flux from an infrared laser are listed in Table 5. The CO laser photons have an average energy of about 0.25 eV; hence, most of these reactions are inaccessible by single-photon processes. If the ionic species possess a long V-T relaxation time, however, then photodissociation or electron photodetachment are possible by multiphoton excitation. Again, because the interaction volume is so restricted, significant modification to an appreciable fraction of the D-layer is not believed possible. This premise should be verified by kinetic modeling which accounts for all plausible processes. Perturbations of this region are known to greatly affect the absorption of hf radio signals and the reflection of If signals; for this reason, the remote possibility of D-region modification by space-to- earth laser power transmission should be investigated further. In the E- and F-regions, the only energetically accessible reaction which was identified is
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