where tm = t-mT, M=t/T, and U(t-t0) is the unit step function. This solution represents the superposition of M 3-dimensional Gaussian-shaped pulses moving away from the point of injection at u = 30 deg long day“l and v = .5 deg long dayl, the width of the pulses increasing as \/t due to diffusive expansion, and the peak amplitude decreasing as fle’^ due to diffusive expansion and photolysis of H2O. Forbes (1980) describes how a water vapor volume mixing ratio (X) representative of the 75-95 Km height region can be inferred from the height integrated number densities. Steady-state values of X for T = .125, .25, 1.0, and 4.0 days at 1 hr and 6 hr after injection are plotted vs. longitude in Figure 1, illustrating that only for T > 1 day to the pulses retain their longitudinal identity without diffusing into one another. This is because the 10% width of the pulses is x/4 Dt loge10 or ~ 10 VF deg (where t is in days), whereas the peak-to-peak spacing is uT in longitude. As shown in Figure 2, the pulses do not retain their identity with respect to latitude since 10 VF >> vT. Note that X is diminished to less than ambient values (~3 ppmv) long before being advected one circuit (360 deg) around the earth. The combined effects of advection by winds, the high mixing rates characteristic of the mesosphere and lower thermosphere, and the short photolytic lifetime of H2O (~2 days), act to prevent significant global or even regional steady-state buildups of H2O. As indicated by Forbes (1980), a baseline value for measurable environmental effects is X = 100 ppmv between 80 and 90 Km, which is only exceeded within an area on the order of 20,000 Km^ (.5 deg lat x 2 deg long) around the point of release. In the lower ionosphere there exists a sharp transition somewhere between 75 and 85 Km where N0+ and 02+ are the dominant positive ions above, and water clusters of the type H+(H20)n (n=l-7) are dominant below. The major source $f molecular ions in the 70 to 90 Km region is photoionization of NO by La(1216A) radiation. Since the cross section of H2O at La is about 1.4 x 10“cm^, a H2O column content of 10"17 molecules cm“2 yields about 75% attenuation of La. For x > 100 ppmv, La radiation reaching the D region is thus reduced by at least 50%. N0+ and 02+ are precursor ions for reactions which lead to formation of H+(H2O)n. It is estimated that values of X exceeding 100 ppmv would lead to a near complete conversion of 02+ and N0+ (with recombination coefficients aj ~ 7 x 10-7 cm3 sec-l) to hydrated ions (012 ~ 3 x 10“$ cm^ sec~l) between 70 and 100 Km. Assuming a square loss law (L=a[e]2) and steady-state conditions, the corresponding reduction in electron density would be no more than (aj/a2)^ 2 0.5. Combined with the La screening effects expected at X = 100 ppmv, a nominal reduction of order 75% in ionization density between 70 and 100 Km can be expected over areas of order 20,000 Km^. One of the net effects of H2O photolysis is to create OH and H. The nature of the formalism adopted here precludes any prediction of the diffusion and redistribution of H atoms in the thermosphere. However, it can be crudely estimated that at least 100 metric tons of H must be added to the natural abundance of 50 metric tons above 105 Km to globally increase the attenuation of La radiation reaching the daytime D-region from the normal 1.6% to 10%, and that
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