CRITICAL ISSUES ON MAGNETIC RECONNECTION IN SPACE PLASMAS A. T. Y. LUI 1 , C. JACQUEY 2 , G. S. LAKHINA 3 , R. LUNDIN 4 , T. NAGAI 5 , T.-D. PHAN 6 , Z. Y. PU 7 , M. ROTH 8 , Y. SONG 9 , R. A. TREUMANN 10 , M. YAMAUCHI 4 and L. M. ZELENYI 11 1 Applied Physics Laboratory, Johns Hopkins Road, Laurel, MD 20723-6099, USA 2 Ctr. d’Etudes Spatiales Rayonnements, 9 ave Colonel Roche, Toulouse 31400, France 3 Indian Institute of Geomagnetism, Colaba, Mumbai 400005, India 4 Swedish Institute of Space Physics, Box 812, S-981 28 Kiruna, Sweden 5 Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo 152-8551, Japan 6 Space Sciences Laboratory, University of California, Berkeley, CA 94720, USA 7 School of Earth and Space Sciences, Peking University, Beijing 100871, China 8 Institut d’A´ eronomie Spatiale de Belgique, Avenue circulaire, 3 B-1180 Brussels, Belgium 9 University of Minnesota, School Physics & Astronomy, Tate Laboratory, 116 Church Street SE, Minneapolis, MN 55455, USA 10 CIPS/MPE, P.O. Box 1312, Garching bei Munich, Munich D-85741, Germany 11 Russian Academy Sciences, Space Research Institute, Profsoyuznaya str. 84/32, Moscow 117810, Russia Abstract. The idea of expedient energy transformation by magnetic reconnection (MR) has generated much enthusiasm in the space plasma community. The early concept of MR, which was envisioned for the solar flare phenomenon in a simple two-dimensional (2D) steady-state situation, is in dire need for extension to encompass three-dimensional (3D) non-steady-state phenomena prevalent in space plasmas in nature like in the magnetosphere. A workshop was organized to address this and related critical issues on MR. The essential outcome of this workshop is summarized in this review. After a brief evaluation on the pros and cons of existing definitions of MR, we propose essentially a working definition that can be used to identify MR in transient and spatially localized phenomena. The word “essentially” reflects a slight diversity in the opinion on how transient and localized 3D MR process might be defined. MR is defined here as a process with the following characteristics: (1) there is a plasma bulk flow across a boundary separating regions with topologically different magnetic field lines if projected on the plane of MR, thereby converting magnetic energy into kinetic particle energy, (2) there can be an out-of-the-plane magnetic field component (the so-called guide field) present such that the reconnected magnetic flux tubes are twisted to form flux ropes, and (3) the region exhibiting non-ideal MHD conditions should be localized to a scale comparable to the ion inertial length in the direction of the plasma inflow velocity. This definition captures the most important 3D aspects and preserves many essential characteristics of the 2D case. It may be considered as the first step in the generalization of the traditional 2D concept. As a demonstration on the utility of this definition, we apply it to identify MR associated with plasma phenomena in the dayside magnetopause and nightside magnetotail of the Earth’s magnetosphere. How MR may be distinguished from other competing mechanisms for these magnetospheric phenomena are then discussed. Space Science Reviews 116: 497–521, 2005. DOI: 10.1007/s11214-005-1987-6 C  Springer 2005