O<sup>+</sup> ion beams reflected below the Martian bow shock: MAVEN observations

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  • K. Masunaga
    Department of Earth and Planetary Science, Graduate School of Science University of Tokyo Tokyo Japan
  • K. Seki
    Department of Earth and Planetary Science, Graduate School of Science University of Tokyo Tokyo Japan
  • D. A. Brain
    Laboratory for Atmospheric and Space Physics University of Colorado Boulder Boulder Colorado USA
  • X. Fang
    Laboratory for Atmospheric and Space Physics University of Colorado Boulder Boulder Colorado USA
  • Y. Dong
    Laboratory for Atmospheric and Space Physics University of Colorado Boulder Boulder Colorado USA
  • B. M. Jakosky
    Laboratory for Atmospheric and Space Physics University of Colorado Boulder Boulder Colorado USA
  • J. P. McFadden
    Space Science Laboratory University of California Berkeley California USA
  • J. S. Halekas
    Department of Physics and Astronomy University of Iowa Iowa City Iowa USA
  • J. E. P. Connerney
    NASA Goddard Space Flight Center Greenbelt Maryland USA

書誌事項

公開日
2016-04
資源種別
journal article
権利情報
  • http://onlinelibrary.wiley.com/termsAndConditions#vor
DOI
  • 10.1002/2016ja022465
公開者
American Geophysical Union (AGU)

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説明

<jats:title>Abstract</jats:title><jats:p>We investigate a generation mechanism of O<jats:sup>+</jats:sup> ion beams observed above the Martian bow shock by analyzing ion velocity distribution functions (VDFs) measured by the Suprathermal and Thermal Ion Composition instrument on the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft. In the solar wind near Mars, MAVEN often observes energetic O<jats:sup>+</jats:sup> ion beams (~10 keV or higher). Accompanied with the O<jats:sup>+</jats:sup> ion beam events, we sometimes observe characteristic ion VDFs in the magnetosheath: a partial ring distribution. The partial ring distribution corresponds to pickup ions with a finite initial velocity (i.e., not newborn pickup ions), and its phase space density is much smaller than that of local pickup O<jats:sup>+</jats:sup> ions of the magnetosheath. Thus, the partial ring distribution is most likely produced by the reflection of pickup O<jats:sup>+</jats:sup> ions precipitating from the upstream solar wind below the bow shock. After being injected into the magnetosheath from the solar wind, the precipitating O<jats:sup>+</jats:sup> ions are subject to the significantly enhanced magnetic field in this region and start to gyrate around the guiding center of the plasma frame in the magnetosheath. Consequently, a part of precipitating O<jats:sup>+</jats:sup> ions are reflected back to the solar wind, generating O<jats:sup>+</jats:sup> beams in the solar wind. The beams direct quasi‐sunward near the subsolar region but have large angle with respect to the sunward direction at high solar zenith angles (>50°). The reflected O<jats:sup>+</jats:sup> beams are accelerated by the convection electric field of the solar wind and may escape Mars.</jats:p>

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