24 and 25 before it can be transported meridionally over a significant distance. For the transport parameter values used in the 2-D model, the mean distance through which material is transported meridionally in 4 days in only —400 km (249 mi). The mesospheric source of nitric oxide includes photochemical processes and reactions that dissociate N2, both EUV and X-rays (38), as well as solar protons, meteorites (28), etc. The 1-D model includes such sources of NO below 120 km (75 mi) and a flux source due to photochemistry of —108 cm“2 sec-1. The NO predictions from the 1-D and 2-D models are in rough qualitative agreement, but differ in details. Because it is not possible to simulate the reentry trajectory in the 1-D model, substantial differences between the results from the two models are expected. Nitric oxide also cools the thermosphere in the 5.3-p.m emission band (39), but the amount of NO produced above 120 km (75 mi) by HLLV reentry is very small, and this effect is expected to be negligible. We have also simulated the dispersion of an NO reentry trail with the aid of the plume model discussed in Sec. 2. Figure 9 illustrates the dispersal of NO at points along the reentry flightpath where NO is generated in large quantities (see Figs. 5 and 6). It is important to note that the profiles in Fig. 9 represent sections along a trajectory that is inclined to the vertical. The horizontal eddy diffusivity employed for the calculations was 108 cm2 sec-1; actually, K is scale-dependent and hence is time-dependent (40), its effective value increasing with time. For periods of less than 1 day, K ~ IO8 cm2 sec-1 is a reasonable value, but it increases rapidly thereafter,
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