Timing of magnetic reconnection initiation during a global magnetospheric substorm onset D. N. Baker, W. K. Peterson, S. Eriksson, and X. Li LASP, University of Colorado, Boulder, Colorado, USA J. B. Blake The Aerospace Corporation, El Segundo, California, USA J. L. Burch Southwest Research Institute, San Antonio, Texas, USA P. W. Daly Max-Planck Institute, Katlenberg, Lindau, Germany M. W. Dunlop Imperial College, London, United Kingdom A. Korth Max-Planck Institute, Katlenberg, Lindau, Germany E. Donovan University of Calgary, Calgary, Canada R. Friedel Los Alamos National Laboratory, Los Alamos, New Mexico, USA T. A. Fritz Center for Space Physics, Boston University, Boston, Massachusetts, USA H. U. Frey and S. B. Mende Space Physics Laboratory, University of California, Berkeley, California, USA J. Roeder The Aerospace Corporation, El Segundo, California, USA H. J. Singer NOAA, Space Environment Center, Boulder, Colorado, USA We have used a unique constellation of Earth-orbiting spacecraft and ground-based measurements in order to study a relatively isolated magnetospheric substorm event on August 27, 2001. Global ultraviolet images of the northern auroral region established the substorm expansion phase onset at 0408:19 (ą1 min) UT. Concurrent measurements from the GOES-8, POLAR, LANL, and CLUSTER spacecraft allow us to construct a timeline which is consistent with magnetic reconnection on the closed field lines of the central plasma sheet near XGSM ~ -18 RE some 7 minutes prior to the near-earth and auroral region times of substorm expansion phase onset. This suggests that magnetic reconnection (i.e., the substorm neutral line) in this case formed in the mid-tail region substantially before current disruption, field dipolarization near geostationary orbit, or auroral substorm onsets occurred. Thus, the magnetic reconnection process is interpreted as the causative driver of dissipation in this well-observed case. _______________ Geophysical Research Letters, 29 (24), 2190, doi:10.1029/2002GL015539, 2002