Charge exchange of metastable ²D oxygen ions with molecular oxygen: A new source of thermospheric O₂⁺ ions

Reactions involving metastable ions are difficult to study in the laboratory. Much new information on these reactions has been derived from satellite measurements of aeronomic parameters. In this paper we use Atmosphere Explorer D data to study charge exchange of metastable O + (² D ) ions with O 2 . Using direct measurements of the O 2 at 200 km to compute O 2 densities at 300 km and supporting ionic concentrations and temperature observations, we find the rate coefficient for this reaction to be 1±0.6 × 10 −9 cm³ s −1 . The process constitutes a significant source of O 2 + ions in the F 2 layer at times when the N 2 and O 2 densities are enhanced. This finding leads to the conclusion that charge exchange with O 2 must be a major sink for O + (² D ) and an important source of O 2 + ions in the E region, because of the increase in the [O 2 ]/[N 2 ] ratio with decreasing altitude. The results imply that 80% of all O + ions formed in the E region are converted to O 2 + and that only ∼20% of the metastable O + ions are converted into N 2 + through charge exchange with N 2 .