where ez is a vertical unit vector, Ke is the eddy diffusivity, and T and H are the temperature and mean atmospheric scale height, respectively. Because the 1-D model extends into the lower thermosphere (i.e., to an altitude of 120 km), molecular diffusion cannot be neglected; a term 0? representing molecular diffusive flux must therefore be added to </>,: mt and m are the molecular mass of the /th constituent and mean molecular mass, respectively, k is the Boltzmann constant, and g is the gravitational acceleration. The 2-D model, which extends only to an altitude of 90 km (56 mi), ignores molecular diffusion. However, because it is a tensor, the eddy diffusion coefficient is more complicated than in the 1-D model; that is, Ke contains “off-diagonal” elements which are important below 25 km (16 mi); the eddy flux is then written The chemical rate coefficients used in the model are essentially those tabulated by Hudson and Reed (8) except that the photolysis rate of NO is taken from the work of Nicolet (9) and Nicolet and Cieslik (10) and three of the odd-hydrogen reaction rate coefficients have been changed (see Sec. 5). Furthermore, we include a source of mesospheric N(2D) (first excited state of N) due to the ionization of NO by solar Lyman-a radiation: In the model, diurnal averaging of photodissociation rates and reaction rate coefficients is performed using, respectively, the techniques reported in Refs. 11 and 12. For each species we specify a lower boundary flux, Ojo, and an upper boundary flux, <I>f„, which may be fixed or have a specified time dependence; at the lower boundary we also include a flux component, by defining the “velocity” at the boundary, vBj, and concentration, nB. (if nB. = 0, our code sets 0B. = 0 automatically); nt is computed by the model. When we use boundary condition specified in Eq. 7, we set the boundary velocity, vB., equal to 1 cm/sec, which is estimated from typical tropospheric mixing rates. A justification for this approach, together with its mathematical application, is given by Turco and Whitten (3). Upper boundary conditions are imposed in a somewhat similar manner, except that allowance must be made for the escape of hydrogen. The mean escape flux of H atoms is about 108 cm-2 sec-1 (13), which must be included in the computa-
RkJQdWJsaXNoZXIy MTU5NjU0Mg==