Analysis and parameterisation of ionic reactions affecting middle atmospheric HO x and NO y during solar proton events
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, HNO 2, H, andOH from N 2 and H 2O. On the other hand, negative ion chemistry redistributes the NO y species, without net production or loss, so that NO, NO 2, and N 2O 5 are converted to HNO 3 and NO 3. 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.