Investigation of the ionospheric D region at sunrise. I. Time variations of ozone, metastable molecular oxygen, and atomic oxygen

As a step in determining the behavior of the D ‐region ionization at sunrise, the development of atomic oxygen and ozone over sunrise has been investigated taking into account the absorption of solar ultraviolet radiation by the time‐varying, nonuniform ozone distribution over the mesosphere at sunrise. The dissociation rates of ozone and molecular oxygen are calculated and used to determine the time variations in the O and O 3 densities at sunrise for a model considering only the oxygen allotropes. Transport processes were neglected in the sunrise calculations because chemical time constants are shorter than those for transport. However, the assumed initial presunrise atomic oxygen and ozone distributions, based on an average of theoretical and experimental results, do include the effects of transport. It was found that at altitudes from 60 to 80 km there is a delay in the ozone sunrise, and thus a delay in the development of the atomic oxygen density. [O]/[O 3 ] was computed during sunrise, and a similar delay in its sunrise increase was observed. The production of metastable oxygen O 2 ( 1 Δ g ,) by the photodissociation of ozone in the Hartley band at sunrise was also investigated. A simplified chemical scheme for O 2 ( 1 Δ) is presented, from which a closed‐form solution for the O 2 ( 1 Δ) densities over sunrise was obtained. O 2 ( 1 Δ) may be important in the ion chemistry at sunrise, and can contribute to the early appearance of electrons in the D ‐region.