Intense focusing of the beam may increase the microwave electric field significantly; an increase in power density by a factor of 100 would be required to approach the theoretical threshold. Although focusing of the microwave beam to this extent is extremely improbable, a process leading to impact ionization of neutrals would be very important if it did occur. The frequency-scaled HF experiments would look for this effect. 2.3.4 Atypical Ionospheric Conditions. Studies of ionosphere-microwave interactions must investigate the physics of potential phenomena for both ideal ionospheric conditions and the disturbed ionosphere. These variations include natural diurnal, seasonal, and solar-cycle changes; atypical ionospheric occurrences such as sporadic-£ and spread-F; and man-made perturbations, i.e., ionospheric depletions following large rocket launches. The last concern may be the greatest, since construction of the solar power satellite would involve frequent launches of heavy-lift launch vehicles. The atmospheric effects of these launches will be addressed below. Large ionospheric density depletions are predicted to accompany the SPS construction (20), and the first solar power satellites will be expected to operate in this modified environment. As a result, the study of ionosphere-microwave interactions must allow for large variations in the unheated ionospheric composition and structure. 3. EMMISSION EFFECTS OF HEAVY LIFT LAUNCH VEHICLES Construction of 2 system of solar power satellites requires many flights of large rockets called heavy lift launch vehicles (HLLV). Powered flights of these large hydrogen, or hydrocarbon, burning rockets could cause temporary removal of ions and electrons from the ionospheric F region. At heights of 150 to 500 km, the normally occurring O+ ions transfer their charge to the combustion products H2O and CO2, forming polyatomic ions that recombine rapidly with free electrons. Severe depletions in ionospheric electron-number density can result. 3.1 Depletion Chemistry The injection of rocket exhaust products, primarily H2O and H2, leads to an enhancement of the effective electron-ion recombination rate through the substitution of polyatomic ions H2O+, H;!O+, and OH+ in place of the normally occurring O+. The depletion effect is apparently confined to the F-layer ionosphere above 200 km altitude where O+ is the dominant positive ion. The main reactions are Reaction 8 is much faster than either of the normally occurring F-layer charge transfer reactions
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