A magnetic reconnection X-line extending more than 390 Earth radii in the solar wind

Phan, T. D.
Gosling, J. T.
Davis, M. S.
Skoug, R. M.
Øieroset, M.
Lin, R. P.
Lepping, R. P.
McComas, D. J.
Smith, C. W.
Reme, H.
Balogh, A.

¹Space Sciences Laboratory, University of California, Berkeley, California 94720, USA. ²Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, Colorado 80303, USA. ³Los Alamos National Laboratory, MS D466, Los Alamos, New Mexico 87545, USA. ⁴Laboratory for Solar and Space Physics, NASA/Goddard Space Flight Center, Greenbelt, Maryland 20771, USA. ⁵Space Science and Engineering Division, Southwest Research Institute, PO Drawer 28510, San Antonio, Texas 78228, USA. ⁶Department of Physics and Space Science Center, University of New Hampshire, Room 207 Morse Hall, 30 College Road, Durham, New Hampshire 03824, USA. ⁷Centre d'Etude Spatiale des Rayonnements, BP 4346, Toulouse 31029, France. ⁸Space and Atmospheric Physics Group, Imperial College, London SW7 2BZ, UK.

doi:10.1038/nature04393

Received 30 August; accepted 31 October 2005.

Author Information Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests. Correspondence and requests for materials should be addressed to T.D.P. ([email protected]).

Nature 439(7073):p 175-178, January 12, 2006.

Magnetic reconnection in a current sheet converts magnetic energy into particle energy, a process that is important in many laboratory, spaceand astrophysical contexts. It is not known at present whether reconnection is fundamentally a process that can occur over an extended region in space or whether it is patchy and unpredictable in nature. Frequent reports of small-scale flux ropes and flow channels associated with reconnectionin the Earth's magnetosphere raise the possibility that reconnection is intrinsically patchy, with each reconnection X-line (the line along which oppositely directed magnetic field lines reconnect) extending at most a few Earth radii (R_E), even though the associated current sheets span many tens or hundreds ofR_E. Here we report three-spacecraft observations of accelerated flow associated with reconnection in a current sheet embedded in the solar wind flow, where the reconnection X-line extended at least 390R_E(or 2.5 × 10⁶km). Observations of this and 27 similar events imply that reconnection is fundamentally a large-scale process. Patchy reconnection observed in the Earth's magnetosphere is therefore likely to be a geophysical effect associated with fluctuating boundary conditions, rather than a fundamental property of reconnection. Our observations also reveal, surprisingly, that reconnection can operate in a quasi-steady-state manner even when undriven by the external flow.