Observations and photochemistry of O⁺⁺ in the daytime thermosphere

A new analysis has been performed of Atmosphere Explorer C observations of O ++ in the daytime thermosphere. The revised chemical model includes the rapid losses of O ++ via N 2 as reported from laboratory measurements and its production from direct double photoionization of O as an additional source. An improved procedure has been introduced for the analysis of the basic ion currents recorded by the magnetic ion‐mass spectrometer and has special application for the very low concentrations near and below 200 km. Large‐scale features in the low‐altitude data are best represented with an additive term proportional to the N 2 (or O 2 ) density, with the preferred interpretation as a contaminant signal induced by neutral particles impinging on the instrument or spacecraft surfaces. This low altitude effect also accounts for otherwise excessive observed amounts of O ++ previously reported near 200 km. The satellite data between 200 and 400 km are consistent with a rate coefficient of 6.6×10 −11 cm³/s for charge‐exchange losses of O ++ to O and with an observational estimate of 2.6×10 −9 s −1 for the production of O ++ from direct double photoionization for conditions near solar minimum. The latter ionospheric result also provides a reference value for the associated basic atomic photoionization cross section.