Abstract

The interpretation of infrared observations of the upper atmosphere is often complicated by the effects of non-LTE (where LTE stands for local thermodynamic equilibrium). This arises whenever collisions between molecules are infrequent enough that the radiative lifetimes of the important vibrational energy states of a molecule become comparable to the mean time between collisions. Although it depends on the state being considered, NLTE typically becomes important at pressures of the order of one mb, or heights above the surface of around 50 km. Then the populations of the internal energy states are no longer related to the kinetic temperature of the gas by simple Boltzmann statistics, but rather by a more complex dependence on radiative exchange of quanta as well as collisional reactions. Under non-LTE conditions, the latter can involve both vibrational - thermal and vibrational-vibrational exchanges, often with a multiplicity of levels and of exchange partners. As a result it becomes quite difficult to perform reliable calculations of the source function, as required to interpret radiance measurements in terms of upper atmospheric temperature structure, or to incorporate the non-LTE energy exchange into dynamical models of the region.

© 1990 Optical Society of America