On the global mean structure of the thermosphere

Abstract

A self‐consistent global average model of the coupled thermosphere and ionosphere has been developed and used to examine the global mean structure of these regions for solar minimum and maximum conditions. The model uses a parameterized version of the solar EUV and UV flux measured from the Atmospheric Explorer satellite to calculate photoionization rates of the major ion species and photo‐dissociation rates for O2 and N2. The ionospheric calculation solves for the vertical distributions of O+, including ambipolar diffusion, and of NO+, O2+, N2+, and N+, assuming photochemical equilibrium with respect to O+. It also solves the electron and ion energy equations as well as the coupled minor neutral constituent diffusion equations for N(²D), N(4S), and NO. The neutral gas energy equation is solved considering heating from photoelectrons, O2 absorption in the Schumann‐Runge continuum and bands, excess energy from exothermic ion‐neutral and neutral‐neutral chemical reactions, thermal electron, ion, and neutral collisions, O3 absorption in the Hartley bands, and atomic oxygen recombination and cooling from molecular and eddy thermal conduction and NO and CO2 IR radiation. The O2 photodissociation rates are used in a model of major species diffusion to solve for the O, O2, and N2 number densities. From specified lower‐boundary conditions and arbitrary initial conditions, both the thermospheric and the ionospheric structure evolve self consistently with time for specified solar flux values. The calculated globally averaged thermospheric and ionospheric structure compares well with the globally averaged structure predicted by the thermospheric and ionospheric empirical models based on data for both solar minimum and maximum conditions by considering solar EUV and UV radiation and aurora particle precipitation and joule heat sources to represent high‐latitude processes. Total joule heating is 7 × 1010 W for geomagnetic quiet conditions for both solar minimum and maximum. The model results show that the dominant heat sources for the thermosphere are collisions between ambient electrons, ions, and neturals above about 250 km, exothermic ion‐neutral chemical reactions between 200 and 250 km, exothermic neutral‐neutral chemical reactions between 200 and 150 km, and heating in the Schumann‐Runge continuum and bands below 150 km. Dominant cooling mechanisms are downward molecular thermal conduction above 150 km and NO and CO2 radiation below 150 km. The results also show that NO cooling must be considered in the thermal energy balance if the effect of minor neutral constituent chemical heating is considered in a definition of the neutral gas solar heating efficiency. Otherwise, the heat input into the thermosphere is overestimated.