Abstract
Abstract. In the polar regions, precipitation of solar high-energy protons and electrons affects the neutral composition of the middle atmosphere. Here we use the Sodankylä Ion and Neutral Chemistry model to calculate ionic production and loss rates of neutral HOx and NOy species, imposed by particle precipitation, for a range of atmospheric conditions and levels of ionization. We also analyse in detail the ionic reaction sequences leading to the HOx and NOy changes. Our results show that particle impact ionization and positive ion chemistry cause net production of N, NO, HNO2, H, andOH from N2 and H2O. On the other hand, negative ion chemistry redistributes the NOy species, without net production or loss, so that NO, NO2, and N2O5 are converted to HNO3 and NO3. Based on the model results, we provide tables of so-called P/Q numbers (i.e. production and loss rates of neutral species divided by ionization rates) at altitudes between 20 and 90 km. These numbers can be easily used to parameterise the ion chemistry effects when modelling atmospheric response to particle precipitation. Compared to earlier studies, our work is the first to consider in detail the NOy effect of negative ion chemistry, and the diurnal and seasonal variability of the P/Q numbers.
Highlights
We have presented an analysis of D region ion chemistry and its effects on neutral HOx and NOy during SPEs
We showed that while positive ion chemistry leads to production of H, OH, HNO2, N, and NO from water vapour and N2, the main effect of negative ion chemistry is to redistribute NOy species, without a net change, by converting NO, NO2, and N2O5 to HNO3 and NO3
The main improvement is the consideration of negative ion chemistry effects, which was previously neglected but is essential for the modelling and understanding of NOy changes induced by particle precipitation
Summary
The Sodankyla Ion and Neutral Chemistry model, known as SIC, is a 1-D tool for ionosphere–neutral atmosphere interaction studies. Because of our attempt to improve the current parameterisation of HOx production, it is worthwhile to briefly explain the main differences in chemistry between SIC and the model used by Solomon et al (1981). For the positive ion scheme, SIC uses more recent reaction rate coefficients and includes a larger number of positive ions. The important improvement is that, in contrast to the Solomon model, SIC includes an extensive negative ion scheme which is important, for example, in understanding the HNO3 production. The details of the ionic reactions included in SIC can be found in Verronen (2006). It should be noted that the SIC model has been compared to satellite-based OH and HNO3 data in several studies of SPEs (Verronen et al, 2006, 2007, 2008, 2011). The reasonably good agreement between the model results and observations found in those studies gives us a confident starting point to this study, which is based solely on the SIC model results
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