High-energy Electron Precipitation Into the atmosphere: an assessment based on balloon-borne observations and model Calculations

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Energetic electrons accelerated in the terrestrial magnetosphere and magnetotail during auroral substorms and geomagnetic storms precipitate into the atmosphere at high latitudes. Primary collision processes with the most abundant species N2, O2, and O lead to the formation of nitric oxides primarily in the mesosphere and upper thermosphere (70—150 km), and start ion- and neutral chemistry reactions affecting many other trace constituents. Nitric oxides can be transported down into the stratosphere below 45 km altitude in large-scale downward motions over polar latitudes during winter, and destroy ozone there in catalytic cycles, the so-called “Energetic particle precipitation indirect effect”. As ozone is one of the key species in radiative heating and cooling of the stratosphere, changes in its composition directly affect stratospheric temperatures and start a chain of dynamical coupling mechanisms affecting atmospheric temperatures and circulation over large areas down to the troposphere. Because of its apparent importance for winter and spring weather systems particularly at high Northern latitudes, it is now recommended, e.g., for model studies initiated by the World Climate Programme WCRP to include energetic particle precipitation in chemistry-climate model studies as part of the natural solar forcing of the climate system. However, recent analyses of the atmospheric ionization rates provided for such model studies based on satellite-based electron flux observations suggest large deficits in these rates, in particular, an underestimation of the atmospheric ionization during and after large geomagnetic storms with high-energy electron precipitation (HEEP) which is particularly large in the stratosphere and lower mesosphere below 60 km.In the frame of this project, we will investigate the impact of HEEP events on the chemical composition, temperatures and dynamics of the middle atmosphere (stratosphere and mesosphere, (10—90 km) by combining atmospheric ionization rates derived from observations of large HEEP events with a long-time balloon data-set going back to 1961 with recent satellite observations of stratospheric and mesospheric trace constituents, and with models of the middle atmosphere of different complexity going from 1-dimensional ion-chemistry, neutral, and radiative-convective models to global chemistry-climate models. In particular, we will address the following two questions:• What is the long-term impact of high-energy electron precipitation on the chemical composition of the middle atmosphere, and due to which processes?• What is the impact of the apparent underestimation of recommended ionization rates as suggested by the recent results on the composition, temperature and dynamics of the middle atmosphere?Results of the project will be of importance for international programs such as WCRP (https://www.wcrp-climate.org), SPARC HEPPA/SOLARIS (http://solarisheppa.geomar.de) and SCOSTEP (http://www.yorku.ca/scostep).