The implication of such a large increase in worldwide thermospheric hydrogen is beyond the scope of the present study, but must be boldly underscored. It has been suggested to the authors by B. M. McCormac (Lockheed Palo Alto Research Laboratory, private communication, 1980) that the population of ring-current protons in the magnetosphere could be substantially altered by charge exchange. Nitric Oxide Nitric oxide deposited in the stratosphere during the launch phase of the HLLV leads to an increase in the global NO abundance of less than 1% over ambient concentrations for 400 launches per year. Nitric oxide is not normally produced in the mesosphere. Rather, it is produced by photochemical processes in the stratosphere and the thermosphere and then transported upward or downward, respectively. The stratospheric source results from the reaction of O('D) with nitrous oxide, and thermospheric nitric oxide is produced through the dissociation of N2, mainly via ionic processes. Intermittent direct mesospheric sources include meteoritic bombardment and auroral activity. However, NO created in the mesosphere and lower thermosphere during HLLV reentry can lead to increases in its upper-atmospheric abundance. The relative magnitude of the increase depends on the background NO concentrations, which are quite uncertain. Figure 8 shows the predicted absolute increases in NO concentrations caused by 10 years of HLLV launches and reentries on the nominal trajectory at the rate of 400 per year. These NO increases are less than 40% at all heights. The “corridor effect” for NO is much more pronounced than was the case for water vapor. The reason for this can be expressed as follows: nitric oxide has a rather short lifetime in the mesosphere, about 4 days at an altitude of 70 km (43 mi) and even shorter at higher altitudes. This is a consequence of rapid photolysis by solar ultraviolet radiation (9,37), followed by the odd-nitrogen destruction reaction, Of course, reaction 25 must compete with the NO recycling reaction whith inhibits NO loss; the rates of reactions 25 and 26 are comparable at an altitude of 70 km (43 mi). The NO lifetime can therefore be expressed as In Eq. 27, k denotes a rate coefficient and J a photolysis rate; values of the reaction rate coefficients were taken from Hudson and Reed (8) and photolysis rates from Nicolet (9). Since the concentration of O2 decreases rapidly with ascending altitude, the lifetime also decreases, reaching an asymptotic value of ~2 days at 90 km (56 mi). One can account for the relatively strong enhancement of NO in the region of rocket reentry by noting that the NO is likely to be destroyed by the sequence of reactions
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