HEPPA III Intercomparison Experiment on Electron Precipitation Impacts: 1. Estimated Ionization Rates During a Geomagnetic Active Period in April 2010

This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.Precipitating auroral and radiation belt electrons are considered an important part of the natural forcing of the climate system. Recent studies suggest that this forcing is underestimated in current chemistry-climate models. The High Energy Particle Precipitation in the Atmosphere III intercomparison experiment is a collective effort to address this point. Here, eight different estimates of medium energy electron (MEE) urn:x-wiley:21699380:media:jgra56926:jgra56926-math-0001 ionization rates are assessed during a geomagnetic active period in April 2010. The objective is to understand the potential uncertainty related to the MEE energy input. The ionization rates are all based on the Medium Energy Proton and Electron Detector (MEPED) on board the NOAA/POES and EUMETSAT/MetOp spacecraft series. However, different data handling, ionization rate calculations, and background atmospheres result in a wide range of mesospheric electron ionization rates. Although the eight data sets agree well in terms of the temporal variability, they differ by about an order of magnitude in ionization rate strength both during geomagnetic quiet and disturbed periods. The largest spread is found in the aftermath of enhanced geomagnetic activity. Furthermore, governed by different energy limits, the atmospheric penetration depth varies, and some differences related to latitudinal coverage are also evident. The mesospheric NO densities simulated with the Whole Atmospheric Community Climate Model driven by highest and lowest ionization rates differ by more than a factor of eight. In a follow-up study, the atmospheric responses are simulated in four chemistry-climate models (CCM) and compared to satellite observations, considering both the CCM structure and the ionization forcing. © 2021. The Authors.This study as well as the companion paper are a collaborative effort of the working group five: Medium Energy Electrons (MEE) Model-Measurement intercomparison of the SPARC Solaris-Heppa initiative, see solarisheppa.geomar.de. The authors thank the SPARC/WCRP for supporting the initial working group meetings. H. Nesse Tyssøy is supported by the Norwegian Research Council (NRC) under contract 223252 and 302040. S. Bender and C. Smith-Johnsen are also supported by the NRC under contract 223252. T. Asikainen is supported by the Academy of Finland (PROSPECT project no: 321440). B. Funke acknowledges financial support from the Agencia Estatal de Investigación of the Ministerio de Ciencia, Innovación y Universidades through projects ESP2017-87 143-R and PID2019-110689RB-I00, as well as the Centre of Excellence “Severo Ochoa” award to the Instituto de Astrofísica de Andalucía (SEV-2017-0709). J. Pettit's work is funded by NSF CEDAR grant AGS 1651 428. E.Rozanov's and T. Sukhodolov's work on the manuscript is done in the SPbSU “Ozone Layer and Upper Atmosphere Research Laboratory” supported by the Ministry of Science and Higher Education of the Russian Federation under agreement 075-15-2021-583 and was partly supported by German Russian cooperation project ”H-EPIC” funded by the Russian Foundation for Basic Research (RFBR project No 20-55-12 020). M. Sinnhuber work was partly supported by the German Research Foundation DFG (grant SI 1088/7-1). The work of P. T. Verronen is supported by the Academy of Finland (project No. 335 555 ICT-SUNVAC). The development of AISstom has been supported by the German Science Foundation (DFG; grant no. WI4417/2-1). J.M. Wissing is supported by the German Aerospace Center (DLR; grant no. D/921/67 284 894). M. Sinnhuber, M. A. Clilverd, B. Funke, C. E. Randall, C. J. Rodger, J. M. Wissing, and P. T. Verronen would like to thank the International Space Science Institute, Bern, Switzerland for supporting the project ”Quantifying Hemispheric Differences in Particle Forcing Effects on Stratospheric Ozone” (Leader: D. R. Marsh).Peer reviewe