The Role of Excited Oxygen Molecules in the Formation of the Secondary Ozone Layer at 87 to 97 km

Abstract

The secondary ozone layer is located at elevations of 87 to 97 km in the upper mesosphere – lower thermosphere. It overlaps with the ionospheric D-layer. Daytime intensive UV radiation is dissociating O2 molecules to O atoms and photoexcitating O2 molecules up to 11.07eV level. Ozone photolysis between the wavelengths of 118.7–121.6 nm produces three oxygen atoms from one ozone molecule. Collision reactions of O2(B3 Σu —) and O2(X3 Σg —, υ≥26) with O2(X3 Σg —, υ=0) produce additional oxygen atoms. The number of oxygen atoms is maintained at such a high level that a small but significant ozone concentration survives. UV radiation weakens radically during the night. The number of O atoms shows no diurnal variation in the MLT. This leads to a ten-fold increase of ozone concentration over the course of the night. Dissociative recombination of O2 + (entered via diffusion from above) and reactions of O (3 P) atoms with excited O2 molecules generate O(1 S) atoms. The quenching of O(1 S)→O(1 D) emits the green nightglow. The reactions of O(1 D) with ozone and O2 absorption of UV nightglow produce O2(c1 Σu —, A’3 ∆u and A3 Σu + ). When these molecules relax, they emit the O2 UV nightglows. The relaxations of O2(a1 ∆g) and O2(b1 Σg + ) emit infrared nightglows.