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Annales Geophysicae An interactive open-access journal of the European Geosciences Union
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Volume 36, issue 2 | Copyright
Ann. Geophys., 36, 445-457, 2018
https://doi.org/10.5194/angeo-36-445-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Regular paper 21 Mar 2018

Regular paper | 21 Mar 2018

Neutralized solar wind ahead of the Earth's magnetopause as contribution to non-thermal exospheric hydrogen

Hans J. Fahr, Uwe Nass, Robindro Dutta-Roy, and Jochen H. Zoennchen Hans J. Fahr et al.
  • Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany

Abstract. In a most recent paper by Qin and Waldrop (2016), it had been found that the scale height of hydrogen in the upper exosphere of the Earth, especially during solar minimum conditions, appears to be surprisingly large. This indicates that during minimum conditions when exobasic temperatures should be small, large exospheric H-scale heights predominate. They thus seem to indicate the presence of a non-thermal hydrogen component in the upper exosphere. In the following parts of the paper we shall investigate what fraction of such expected hot hydrogen atoms could have their origin from protons of the shocked solar wind ahead of the magnetopause converted into energetic neutral atoms (ENAs) via charge-exchange processes with normal atmospheric, i.e., exospheric hydrogen atoms that in the first step evaporate from the exobase into the magnetosheath plasma region. We shall show that, dependent on the sunward location of the magnetopause, the density of these types of non-thermal hydrogen atoms (H-ENAs) becomes progressively comparable with the density of exobasic hydrogen with increasing altitude. At low exobasic heights, however, their contribution is negligible. At the end of this paper, we finally study the question of whether the H-ENA population could even be understood as a self-consistency phenomenon of the H-ENA population, especially during solar activity minimum conditions, i.e., H-ENAs leaving the exosphere being replaced by H-ENAs injected into the exosphere.

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We investigate what fraction of the hot hydrogen atoms recently found from Lyman-alpha measurements in the Earth's upper exosphere could have their origin as energetic neutral atoms via charge exchange from protons of the shocked solar wind ahead of the magnetopause. Our calculations show that this contribution, although definitely present at larger exospheric heights, cannot explain the observations at lower altitudes.
We investigate what fraction of the hot hydrogen atoms recently found from Lyman-alpha...
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