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Science 4 July 2008:
Vol. 321. no. 5885, pp. 85 - 89
DOI: 10.1126/science.1159040
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Reports

Mercury's Magnetosphere After MESSENGER's First Flyby

James A. Slavin,1* Mario H. Acuña,2 Brian J. Anderson,3 Daniel N. Baker,4 Mehdi Benna,2 George Gloeckler,5,6 Robert E. Gold,3 George C. Ho,3 Rosemary M. Killen,6 Haje Korth,3 Stamatios M. Krimigis,3,7 Ralph L. McNutt, Jr.,3 Larry R. Nittler,8 Jim M. Raines,5 David Schriver,9 Sean C. Solomon,8 Richard D. Starr,10 Pavel Trávnícek,11 Thomas H. Zurbuchen5

Observations by MESSENGER show that Mercury's magnetosphere is immersed in a comet-like cloud of planetary ions. The most abundant, Na+, is broadly distributed but exhibits flux maxima in the magnetosheath, where the local plasma flow speed is high, and near the spacecraft's closest approach, where atmospheric density should peak. The magnetic field showed reconnection signatures in the form of flux transfer events, azimuthal rotations consistent with Kelvin-Helmholtz waves along the magnetopause, and extensive ultralow-frequency wave activity. Two outbound current sheet boundaries were observed, across which the magnetic field decreased in a manner suggestive of a double magnetopause. The separation of these current layers, comparable to the gyro-radius of a Na+ pickup ion entering the magnetosphere after being accelerated in the magnetosheath, may indicate a planetary ion boundary layer.

1 Heliophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA.
2 Solar System Exploration Division, NASA Goddard Space Fight Center, Greenbelt, MD 20771, USA.
3 Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA.
4 Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO 80303, USA.
5 Department of Atmospheric, Oceanic, and Space Sciences, University of Michigan, Ann Arbor, MI 48109, USA.
6 Department of Astronomy, University of Maryland, College Park, MD20742, USA.
7 Academy of Athens, Athens 11527, Greece.
8 Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015, USA.
9 Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90024, USA.
10 Department of Physics, Catholic University of America, Washington, DC 20064, USA.
11 Astronomical Institute, Academy of Sciences of the Czech Republic, Prague, Czech Republic.

* To whom correspondence should be addressed. E-mail: james.a.slavin{at}nasa.gov

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