results are summarized in a recent report by Zinn, Sutherland, Smith, and Pongratz (Ref. 3.7). With respect to the airglow observations at LAGOPEDO, Pongratz and Smith (Ref. 3.8) reported it was still detectable after 400 sec. Sutherland and Zinn (Ref. 3.5) were able to fit the time-dependence of the observed airglow reasonably well with their ionospheric depletion model calculations but the spatial resolution is uncertain, since the data have not yet been reduced (this is being done as part of the SPS assessment program). Zinn et al. (Ref. 3.7) have pointed out that since their model does not include any treatment of condensation and evaporation, the simulation of processes involving water vapor is probably incorrect. They also mention, however, that since reactions involving carbon dioxide produce more than 50% of the airglow, the predicted airglow intensity is not very sensitive to water vapor chemistry. Sjolander and Johnson (Ref. 3.9), on the other hand, described the expanding cloud through a simple spherical diffusion equation that incorporated a simple treatment of H2O condensation and evaporation. They estimated that the snow* disappeared after about 80 sec. Their calculations for 0+, H20+, N0+ ions compared well with observations but their H30+ results did not. Sjolander and Johnson found that the CO+ data could be fitted without CO2 condensation, in agreement with Zinn et al. (Ref. 3.7). Hence, both the inadvertant Skylab II ionospheric depletion and the deliberate LAGOPEDO experiments demonstrated that the ionosphere can be depleted and that plausible mechanisms exist to explain the phenomena involved (at least in an approximate manner). What remains to be determined is what the nature and extent of this depletion is and how it will scale with increased rocket size and frequency of launches, and what the consequences are, Zinn et al. (Ref. 3.7) have carried out some preliminary calculations with a one-dimensional ionospheric chemistry model that suggest that daily launches of the type of HLLV envisioned in the SPS transportation system will result in continuous, substantial reduction in the total ionsphere. *Ice crystals formed initially during the rapid expansion and cooling of the water vapor cloud following detonation.
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