effects on ionization in the F-layer are, as we have shown, fairly small. However, relatively small but significant amounts of H2O and H2 are deposited directly into the F-layer during the orbit circularization and deorbit maneuvers. The rates of exhaust emission per linear kilometer during those maneuvers are expected to be about one-tenth of the emission rate of the Saturn V, S II engines during the Skylab launch. However, since the bum would occur at a high altitude (477 km), the efficiency for ion removal would be high. We ran a computation to represent an HLLV orbit circularization bum occurring at noon over the Philippine Islands. The assumed exhaust deposition rate was 4.9x 1026 H2O molecules and 1.6x 1026 H2 molecules per kilometer, extending over a linear distance of 1400 km. (These parameters were furnished by H.P. Davis, NASA JSC, based on expected characteristics of the HLLV orbital-maneuvering system.) After injection, the gases were assumed to fall rapidly to 270-km, while spreading crosswise to the trajectory plane over a perpendicular distance of 400 km (full width). According to the computation, a small but definite ionospheric hole is produced, illustrated in Figs 7a, b, and c. These figures represent a cross section perpendicular to the vehicle trajectory and lying in the geomagnetic meridian plane. The time is 2.4 h after launch, the time of maximum ion depletion. The electron density has been reduced to one-third of its normal value over a geographic area approximately 1000 by 2000 km. The ionosphere returns to normal in about 5 h. 6. THE COMPUTER MODELS Two-dimensional Cartesian model Structurally, the code includes an array of two-dimensional Eulerian mesh ceils with coordinates x (in the geomagnetic meridian plane) and z (the vertical). The z-coordinate extends from 50- to 700-km altitude; the x-coordinate extends from 0 to 2000 km on either side of the trajectory plane. There are 13 vertical and 21 horizontal cells, for a total of 273 cells. The following physical and chemical processes are considered. a. Chemistry. Time-dependent chemical/photochemical kinetics with 28 species and 277 reactions. The rate equations are integrated in each of the 273 cells. The more important chemical reactions in the reaction set are listed in Appendix A. b. Solar radiation. The nominal spectral intensity of sunlight in each of 40 wavelength bands is stored by the computer code. To simulate expected solar-activity-related variations in the EUV region below 1000 A, the nominal intensities are scaled up or down by a nominal factor, usually in the range 0.5 to 2. Attenuation of the solar radiation in each wavelength band at each of the 13 altitudes due to absorption by O, O2, N2, and O3 is computed from the computed concentrations of those species and the computed solar zenith angles. The rate coefficients for photodissociation, photoionization, and photoexcitation reactions are computed from the computed spectral intensitis and tabulated wavelength-dependent cross sections. c. Diffusion and transport. Diffusion rates are computed for each species under the influence of gravity under assumed conditions of steady flow (near hydrostatic equilibrium). Advection effects due to horizontal winds are included. The
RkJQdWJsaXNoZXIy MTU5NjU0Mg==