Composition changes after the "Halloween" solar proton event: The High Energy Particle Precipitation in the Atmosphere (HEPPA) model versus MIPAS data intercomparison study

Simulated SPE-induced ozone losses agree on average within 5 % with the observations. Simulated NOy enhancements around 1 hPa, however, are typically 30 % higher than indicated by the observations which are likely to be related to deficiencies in the used ionization rates, though other error sources related to the models' atmospheric background state and/or transport schemes cannot be excluded. The analysis of the observed and modeled NOy partitioning in the aftermath of the SPE has demonstrated the need to implement additional ion chemistry (HNO₃ formation via ion-ion recombination and water cluster ions) into the chemical schemes.

An overestimation of observed H₂O₂ enhancements by all models hints at an underestimation of the OH/HO₂ ratio in the upper polar stratosphere during the SPE. The analysis of chlorine species perturbations has shown that the encountered differences between models and observations, particularly the underestimation of observed ClONO₂ enhancements, are related to a smaller availability of ClO in the polar night region already before the SPE.

In general, the intercomparison has demonstrated that differences in the meteorology and/or initial state of the atmosphere in the simulations cause a relevant variability of the model results, even on a short timescale of only a few days.