A detailed evaluation of the heating efficiency in the middle atmosphere

Abstract

The conversion to heat of solar ultraviolet radiation absorbed by ozone and molecular oxygen in the terrestrial mesosphere and lower thermosphere occurs through a series of complex processes. Upon photolysis of the O 3 or O 2, significant amounts of chemical potential energy and atomic and molecular internal energy are generated. The disposition of the internal energy largely determines the rate at which the atmosphere is heated. In addition, the chemical energy is released subsequent to exothermic chemical reactions which may occur long after and far away from the location of photon deposition. Energy may be lost from the atmosphere by airglow from excited photolysis products or by chemiluminescent emission from product species of exothermic chemical reactions. In this paper we examine the role of airglow losses in reducing the efficiency of solar heating in the Hartley, Huggins, and Chappuis bands of ozone and in the Herzberg, Ly α, Schumann‐Runge continuum, and Schumann‐Runge bands of molecular oxygen. We also examine the role of heating due to seven chemical reactions and calculate the efficiencies for those reactions with significant chemiluminescent loss. Parameterizations of the heating efficiency that are readily applicable to numerical models are given for those processes with nonunit efficiencies. Results from the calculation of heating rates for individual processes indicate that the reaction of atomic hydrogen and ozone is potentially the largest single source of heat in the vicinity of the mesopause. However, significant improvement is still needed in the knowledge of the quenching and chemical reaction rates of vibrationally excited OH before the efficiency of this reaction can be confidently calculated. Our calculations also indicate that even under strong quenching, most of the OH is in vibrationally excited form above 85–90 km. Finally, the bulk heating efficiency due to the combination of solar and chemical heating is calculated. The calculated bulk efficiencies demonstrate that airglow and chemiluminescent emission significantly reduce the amount of energy available for heat throughout the mesosphere and lower thermosphere.