Sunset transition of negative charge in the D-region ionosphere during high-ionization conditions
Abstract. The solar proton event of October 1989 and especially the sunset of 23 October is examined in this study of negative ion chemistry, which combines measurements of nitric oxide, electron density, and cosmic radio noise absorption with ion and neutral chemistry modelling. Model results show that the negative charge transition from electrons to negative ions during sunset occurs at altitudes below 80 km and is dependent on both ultraviolet and visible solar radiation. The ultraviolet effect is mostly due to rapid changes in atomic oxygen and O2(1Δg), while the decrease in NO3- photodetachment plays a minor role. The effect driven by visible wavelengths is due to changes in photodissociation of CO3- and the subsequent electron photodetachment from O-, and at higher altitudes is also due to a decrease in the photodetachment of O2-. The relative sizes of the ultraviolet and visible effects vary with altitude, with the visible effects increasing in importance at higher altitudes, and they are also controlled by the nitric oxide concentration. These modelling results are in good agreement with EISCAT incoherent scatter radar and Kilpisjärvi riometer measurements.