Cold ions in the hot plasma sheet of Earth's magnetotail

Kanako Seki (), Masafumi Hirahara, Masahiro Hoshino, Toshio Terasawa, Richard C. Elphic, Yoshifumi Saito, Toshifumi Mukai, Hajime Hayakawa, Hirotsugu Kojima and Hiroshi Matsumoto
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Kanako Seki: Nagoya University
Masafumi Hirahara: Rikkyo University
Masahiro Hoshino: University of Tokyo
Toshio Terasawa: University of Tokyo
Richard C. Elphic: NIS-1, MS D466, Los Alamos National Laboratory
Yoshifumi Saito: Institute of Space and Astronautical Science
Toshifumi Mukai: Institute of Space and Astronautical Science
Hajime Hayakawa: Institute of Space and Astronautical Science
Hirotsugu Kojima: Kyoto University
Hiroshi Matsumoto: Kyoto University

Nature, 2003, vol. 422, issue 6932, 589-592

Abstract: Abstract Most visible matter in the Universe exists as plasma. How this plasma is heated, and especially how the initial non-equilibrium plasma distributions relax to thermal equilibrium (as predicted by Maxwell–Boltzman statistics), is a fundamental question in studies of astrophysical1,2,3 and laboratory plasmas4,5. Astrophysical plasmas are often so tenuous that binary collisions2,3 can be ignored, and it is not clear how thermal equilibrium develops for these ‘collisionless’ plasmas. One example of a collisionless plasma is the Earth's plasma sheet6, where thermalized hot plasma with ion temperatures of about 5 × 107 K has been observed7. Here we report direct observations of a plasma distribution function during a solar eclipse, revealing cold ions in the Earth's plasma sheet in coexistence with thermalized hot ions. This cold component cannot be detected by plasma sensors on satellites that are positively charged in sunlight, but our observations in the Earth's shadow show that the density of the cold ions is comparable to that of hot ions. This high density is difficult to explain within existing theories8,9,10, as it requires a mechanism that permits half of the source plasma to remain cold upon entry into the hot turbulent plasma sheet.

Date: 2003
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DOI: 10.1038/nature01502

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