Quenching of O+/2D/ by electrons in the thermosphere

Abstract A major loss process for the metastable species, O + ( 2 D), in the thermosphere is quenching by electrons O + ( 2 D) + e → O + ( 4 S) + e . To date no laboratory measurement exists for the rate coefficient of this reaction. Thermospheric models involving this process have thus depended on a theoretically calculated value for the rate coefficient and its variation with electron temperature. Earlier studies of the O + ( 2 D) ion based on the Atmosphere Explorer data gathered near solar minimum, could not quantify this process. However, Atmosphere Explorer measurements made during 1978 exhibit electron densities that are significantly enhanced over those occurring in 1974, due to the large increases that have occurred in the solar extreme ultraviolet flux. Under such conditions, for altitudes ≳ 280 km, the electron quenching process becomes the major loss mechanism for O + ( 2 D), and the chemistry of the N + 2 ion, from which the O + ( 2 D) density is deduced, simplifies to well determined processes. We are thus able to use the in situ satellite measurements made during 1978 to derive the electron quenching rate coefficient. The results confirm the absolute magnitude of the theoretical calculation of the rate coefficient, given by the analytical expression k ( T e ) = 7.8 × 10 −8 ( T e /300) −0.5 cm 3 s −1 . There is an indication of a stronger temperature dependence, but the agreement is within the error of measurement.