(e) Scattered Lyman-a and Lyman-B enhancements due to augmentation of the hydrogen geocorona, leading to greater nighttime ionization in the D- and lower E-regions. (f) Atmospheric conductivity profile alterations resulting from changes in ion and elctron concentration and composition. (4) Thermal and dynamical perturbations near the mesopause following large H2O/H2 injections. The major thermal effects are probably related to direct infrared radiation transfer in the water vapor bands (between 6.3 and 2.7 pm), OH infrared emission in the Meinel bands (between about 0.6 and 3.0 pm), and chemical energy release by hydrogen-catalyzed oxygen atom recombination. Note that the OH Meinel band emissions can represent up to 20% of the ambient cooling rate of the mesopause region. It is not possible to estimate the dynamical implications of small rocket-induced temperature changes because the dynamics of the mesosphere are only poorly understood at present. Although the effects outlined above will probably be small on a global scale, locally (i.e., within 5-10° latitude of the injection) the effects may be large enough to cause noticeable (and possibly detrimental), changes. Accordingly, it is important to consider both the short-term local, and longterm global, impacts of SPS rocket activities. 2.9 IONOSPHERIC CONDUCTIVITY AND ATMOSPHERIC ELECTRICITY (Vondrak) The electrical conductivity of the ionosphere and middle atmosphere is directly proportional to the electron and ion concentration. The injection of H2O and H2 by the SPS transportation system will cause both localized and widespread reduction in ionization density and in conductivity. On the other hand, the NO produced during reentry is expected to increase the D-region electron density, resulting in a localized conductivity enhancement. A global increase in nightttime D- and E-region ionization might also result from Lyman-a and Lyman-B scattering from an enhanced hydrogen geocorona (see Section 3.1.4). Another possible source of conductivity change is an alteration of charged particle precipitation at high latitudes if the SPS transportation system alters the radiation belts or magnetospheric structure (see Sections 4.3-4.5). Particle precipitation is the main source of the high-latitude night time ionosphere and, if the particles are energetic, it may enhance the conductivity of the middle atmosphere. An alteration of the ionospheric conductivity will affect the pattern of the ionospheric currents and electric fields. In particular, the intensitv of the equatorial and low latitude electric field is influenced by the ionospheric conductivity distribution at high latitudes. An alteration of the equatorial electric field may strongly affect the occurrence of equatorial Spread F. At higher latitudes the ionospheric electric currents are part of the electric current circuit that includes the Birkeland currents coupling the ionosphere to the magnetosphere. Thus modifying the high-latitude conductivity may alter the magnetospheric structure. The conductivity in the middle atmosphere is important for communication systems, particularly VLF. Also, changes in electrical conductivity in
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