Analysis and parameterisation of ionic reactions affecting middle atmospheric HO_x and NO_y during solar proton events

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 HO_x and NO_y 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 HO_x and NO_y 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 NO_y 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 NO_y effect of negative ion chemistry, and the diurnal and seasonal variability of the P/Q numbers.