T I Pulkkinen, D N Baker, N E Turner, 
Laboratory for Atmospheric and Space Physics,
University of Colorado,
Campus Box 590, Boulder, CO 80301
tel. 303-492 4831
e-mail tuija.pulkkinen@lasp.colorado.edu

L A Frank, J B Sigwarth, 
Department of Physics and Astronomy,
The University of Iowa, 
Iowa City, IA 

H Singer, 
NOAA Space Environment Center,
Boulder, CO 

G D Reeves, 
Los Alamos National Laboratory,
Los Alamos, NM 

J B Blake,
The Aerospace Corporation, 
Los Angeles, CA

T Mukai,
Institute of Space and Astronautical Science, 
Sagamihara, Japan

S Kokubun, and R Nakamura
Solar Terrestrial Environment Laboratory,
Nagoya University,
Toyokawa, Japan

On May 15, 1996, two sequential substorm onsets took place about 40 min apart following a southward turning of the IMF. The events were observed by several ISTP spacecraft in the upstream solar wind, by several spacecraft in the magnetotail, and by an extensive complement of ground-based instruments covering a wide range of latitudes and local times. The first event occurred during persistently negative IMF Bz, whereas the second onset occurred at the time when the IMF changed from southward to northward orientation. The two events had very different global evolution: The first onset led only to minor reconfiguration of the tail field and a short-lived auroral intensification. On the other hand, the second onset caused much stronger field dipolarization, global auroral expansion, and significant polar cap area reduction. Magnetic field modeling and field-aligned mappings are used to find the source regions of both the auroral brightenings and the substorm current wedge currents, which in this event were not colocated. The significance of the solar wind/IMF parameters and ionospheric conditions to the substorm evolution after the initial onset are discussed.