3 EFFECTS OF HYDROGEN AND WATER INJECTIONS ON THE IONOSPHERE 3.1 PHENOMENOLOGY OF HYDROGEN IN THE UPPER ATMOSPHERE 3.1.1 The Overall Problem The atmosphere of the earth as a whole loses hydrogen atoms by diffusion at a rate on the order of 10J^ atoms/year (see Donahue, 1977). Presumably most of this hydrogen comes from water or methane transported from the ground into the stratosphere and then dissociated by chemical and photochemical reactions. The loss rate of hydrogen is not uniform — for exospheric temperatures below 800 K, there may be little loss, and indeed there is a discrepancy by a factor of 4-5 between the water burden in the stratosphere and the escape flux — but the general flow of hydrogen does appear to be upward, out of the atmosphere. The SPS annual injection rate of hydrogen in the atmosphere above 75 km is of the order of 10J atoms/year (see Table 3), and if a significant fraction of it leaves the atmosphere, a non-negligible perturbation to the atmospheric loss of hydrogen may be produced. What is the fate of the injected hydrogen? and what effects may be anticipated from this enhanced loss rate? This problem is addressed in Section 3.1.2, following. Most of the hydrogen injected is deposited below 120 km (97% of the total, see Table 3). Of this hydrogen, three quarters comes out as water, that mixes with ambient mesospheric water, as reviewed in Section 2, but one quarter is emitted as H2 , and much of this diffuses upward and can give rise to permanent ionospheric depletion. The depletion problem is reviewed in Section 3.2, where the possibility of a major ionospheric depletion arising from upward transport of H2 (and possibly also H2O) from the HLLV second stage is discussed. The rest of the water, which is emitted at LEO, in the passage to, or at, GEO, is considered in detail in Section 3.1.3. After ejection, roughly half of it, or about 1.5% of the total water emitted, falls under gravity to altitudes of 120-150 km, where it presumably mixes with the bulk of upper atmospheric water. Of the total amount of water emitted at GEO, about 0.9% is trapped in long-lasting orbits; about 0.7% may either escape or fall, depending on the rate that the ice crystals formed in expansion from the rocket nozzle evaporate (to permit the resulting molecules to equilibrate by collisions with the ambient atmosphere); and about 0.1% will escape from the earth's atmosphere. Note that the specific problems of H2O/H2 emission in the magnetosphere are discussed in Section 4.2. 3.1.2 Fate of H2O/H2 Injected in the Thermosphere (Zinn) The ultimate fate of the exhaust molecules is to form hydrogen atoms that escape from the atmosphere. SPS activities will increase this rate of escape, possibly by as much as a factor of 2. This in itself is not serious, since as far as we know hydrogen atoms have always been escaping from the
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