Abstract
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.
Highlights
Negative ions are a feature of the D-region ionosphere, where they hold a substantial portion of the negative charge
We found that a combination of 650-keV energy and 1.45×109 m−2 s−1 flux resulted in electron density levels quite close to the EISCAT measurements
A 1-D neutral and ion chemistry model has been used to interpret measurements of nitric oxide, cosmic radio noise absorption, and electron density made during the great proton storm of October 1989, in high ionization conditions
Summary
Negative ions are a feature of the D-region ionosphere, where they hold a substantial portion of the negative charge. The transition of negative charge is driven by solar light Of those ions that are neutralised by photons, only NO−3 has such a high electron affinity that ultraviolet (UV) radiation is required (Collis and Rietveld, 1990, and references therein). T. Verronen et al.: Negative charge transition in the D-region during sunset and references therein for more information on neutral atmospheric effects). The latest version solves time-dependent concentrations of 63 ions, including 29 negative ions, as well as 13 neutral species (O(3P), O(1D), O3, N(4S), N(2D), NO, NO2, NO3, HNO3, N2O5, H, OH, and HO2) between the altitudes of 20 and 150 km, with a 1-km resolution, taking into account several hundred chemical reactions and external forcing due to solar radiation (1−422 nm), electron and proton precipitation, and galactic cosmic rays. The other reactions important to H2O2 were already included in the model
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