Cyclic reformation of a quasi-parallel bow shock at Mercury: MESSENGER observations

Authors

  • Torbjörn Sundberg,

    Corresponding author
    1. Center for Space Physics, Boston University, Boston, Massachusetts, USA
    2. Heliophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
    3. School of Physics and Astronomy, Queen Mary University of London, London, UK
    • Corresponding author: T. Sundberg, School of Physics and Astronomy, Queen Mary University of London, London E1 4NS, UK. (torbjorn.sundberg@gmail.com)

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  • Scott A. Boardsen,

    1. Heliophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
    2. Goddard Earth Sciences and Technology Center, University of Maryland, College Park, Maryland, USA
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  • James A. Slavin,

    1. Department of Atmospheric, Oceanic and Space Sciences, University of Michigan, Ann Arbor, Michigan, USA
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  • Vadim M. Uritsky,

    1. Heliophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
    2. Catholic University of America, Washington, D.C., USA
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  • Brian J. Anderson,

    1. The Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA
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  • Haje Korth,

    1. The Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA
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  • Daniel J. Gershman,

    1. Department of Atmospheric, Oceanic and Space Sciences, University of Michigan, Ann Arbor, Michigan, USA
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  • Jim M. Raines,

    1. Department of Atmospheric, Oceanic and Space Sciences, University of Michigan, Ann Arbor, Michigan, USA
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  • Thomas H. Zurbuchen,

    1. Department of Atmospheric, Oceanic and Space Sciences, University of Michigan, Ann Arbor, Michigan, USA
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  • Sean C. Solomon

    1. Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York, USA
    2. Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, D.C., USA
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Abstract

[1] We here document with magnetic field observations a passage of the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft through Mercury's magnetosphere under conditions of a quasi-parallel bow shock, i.e., when the direction of the upstream interplanetary magnetic field was within 45° of the bow shock normal. The spacecraft's fast transition of the magnetosheath and the steady solar wind conditions during the period analyzed allow both spatial and temporal properties of the shock crossing to be investigated. The observations show that the shock reformation process can be nearly periodic under stable solar wind conditions. Throughout the 25-min-long observation period, the pulsation duration deviated by at most ~10% from the average 10 s period measured. This quasiperiodicity allows us to study all aspects of the shock reconfiguration, including ultra-low-frequency waves in the upstream region and large-amplitude magnetic structures observed in the vicinity of the magnetosheath-solar wind transition region and inside the magnetosheath. We also show that bow shock reformation can be a substantial source of wave activity in the magnetosphere, on this occasion having given rise to oscillations in the magnetic field with peak-to-peak amplitudes of 40–50 nT over large parts of the dayside magnetosphere. The clean and cyclic behavior observed throughout the magnetosphere, the magnetosheath, and the upstream region indicates that the subsolar region was primarily influenced by a cyclic reformation of the shock front, rather than by a spatial and temporal patchwork of short large-amplitude magnetic structures, as is generally the case at the terrestrial bow shock under quasi-parallel conditions.

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