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Annales Geophysicae An interactive open-access journal of the European Geosciences Union
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Volume 33, issue 3 | Copyright
Ann. Geophys., 33, 381-394, 2015
https://doi.org/10.5194/angeo-33-381-2015
© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.

Regular paper 23 Mar 2015

Regular paper | 23 Mar 2015

Contribution of proton and electron precipitation to the observed electron concentration in October–November 2003 and September 2005

P. T. Verronen1, M. E. Andersson1, A. Kero2, C.-F. Enell3, J. M. Wissing4, E. R. Talaat5,*, K. Kauristie1, M. Palmroth1, T. E. Sarris6, and E. Armandillo7 P. T. Verronen et al.
  • 1Earth Observation, Finnish Meteorological Institute, Helsinki, Finland
  • 2Sodankylä Geophysical Observatory, University of Oulu, Sodankylä, Finland
  • 3EISCAT Scientific Association, Kiruna, Sweden
  • 4Institute of Environmental Systems Research, University of Osnabrück, Osnabrück, Germany
  • 5The Johns Hopkins University Applied Physics Laboratory, Laurel, MD, USA
  • 6Space Research Laboratory, Democritus University of Thrace, Xanthi, Greece
  • 7European Space Agency, ESTEC, Nordwijk, the Netherlands
  • *present address: NASA Headquarters, Washington, D.C., USA

Abstract. Understanding the altitude distribution of particle precipitation forcing is vital for the assessment of its atmospheric and climate impacts. However, the proportion of electron and proton forcing around the mesopause region during solar proton events is not always clear due to uncertainties in satellite-based flux observations. Here we use electron concentration observations of the European Incoherent Scatter Scientific Association (EISCAT) incoherent scatter radars located at Tromsø (69.58° N, 19.23° E) to investigate the contribution of proton and electron precipitation to the changes taking place during two solar proton events. The EISCAT measurements are compared to the results from the Sodankylä Ion and Neutral Chemistry Model (SIC). The proton ionization rates are calculated by two different methods – a simple energy deposition calculation and the Atmospheric Ionization Model Osnabrück (AIMOS v1.2), the latter providing also the electron ionization rates. Our results show that in general the combination of AIMOS and SIC is able to reproduce the observed electron concentration within ± 50% when both electron and proton forcing is included. Electron contribution is dominant above 90 km, and can contribute significantly also in the upper mesosphere especially during low or moderate proton forcing. In the case of strong proton forcing, the AIMOS electron ionization rates seem to suffer from proton contamination of satellite-based flux data. This leads to overestimation of modelled electron concentrations by up to 90% between 75–90 km and up to 100–150% at 70–75 km. Above 90 km, the model bias varies significantly between the events. Although we cannot completely rule out EISCAT data issues, the difference is most likely a result of the spatio-temporal fine structure of electron precipitation during individual events that cannot be fully captured by sparse in situ flux (point) measurements, nor by the statistical AIMOS model which is based upon these observations.

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Electron concentrations observed by EISCAT radars can be reasonable well represented using AIMOS v1.2 satellite-data-based ionization model and SIC D-region ion chemistry model. SIC-EISCAT difference varies from event to event, probably because the statistical nature of AIMOS ionization is not capturing all the spatio-temporal fine structure of electron precipitation. Below 90km, AIMOS overestimates electron ionization because of proton contamination of the satellite electron detectors.
Electron concentrations observed by EISCAT radars can be reasonable well represented using AIMOS...
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