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Originally published in Science Express on 24 July 2008
Science 15 August 2008:
Vol. 321. no. 5891, pp. 931 - 935
DOI: 10.1126/science.1160495
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Research Articles

Tail Reconnection Triggering Substorm Onset

Vassilis Angelopoulos,1* James P. McFadden,2 Davin Larson,2 Charles W. Carlson,2 Stephen B. Mende,2 Harald Frey,2 Tai Phan,2 David G. Sibeck,3 Karl-Heinz Glassmeier,4 Uli Auster,4 Eric Donovan,5 Ian R. Mann,6 I. Jonathan Rae,6 Christopher T. Russell,1 Andrei Runov,1 Xu-Zhi Zhou,1 Larry Kepko7

Magnetospheric substorms explosively release solar wind energy previously stored in Earth's magnetotail, encompassing the entire magnetosphere and producing spectacular auroral displays. It has been unclear whether a substorm is triggered by a disruption of the electrical current flowing across the near-Earth magnetotail, at ~10 RE (RE: Earth radius, or 6374 kilometers), or by the process of magnetic reconnection typically seen farther out in the magnetotail, at ~20 to 30 RE. We report on simultaneous measurements in the magnetotail at multiple distances, at the time of substorm onset. Reconnection was observed at 20 RE, at least 1.5 minutes before auroral intensification, at least 2 minutes before substorm expansion, and about 3 minutes before near-Earth current disruption. These results demonstrate that substorms are likely initiated by tail reconnection.

1 Institute of Geophysics and Planetary Physics/ESS, University of California at Los Angeles, Los Angeles, CA 90095, USA.
2 Space Sciences Laboratory, University of California at Berkeley, CA 94720, USA.
3 Code 674, NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA.
4 TUBS, Braunschweig, D-38106, Germany.
5 Department of Physics and Astronomy, University of Calgary, Calgary, Alberta, Canada T2N 1N4.
6 Department of Physics, University of Alberta, Edmonton, Alberta, Canada T6G 2M7.
7 Space Science Center, University of New Hampshire, Durham, NH 03824, USA.

* To whom correspondence should be addressed. E-mail: vassilis{at}ucla.edu

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Science. ISSN 0036-8075 (print), 1095-9203 (online)