The physics of substorms as revealed by the ISTP

Spacecraft and ground data combined with multiscale computer models develop ed by the ISTP program are providing a new and coherent understanding of th e magnetospheric substorms and storms. Global MHD simulations that include ionospheric response are dynamically driven by upstream satellite data and allow for direct comparison with the field and flow quantities measured by magnetospheric satellites, ground data and images from the POLAR satellite. Through the combined analysis of the simulations and observations, the fir st unified picture of a substorm from the magnetospheric and ionospheric vi ewpoint is currently emerging. Here we use MHD simulations of two particula rly well observed and analyzed events to explore the factors that trigger a nd organize the substorm elements into a coherent entity. The first event - March 9, 1995 - produces clear evidence that impulsive penetration of a la rge electric field in the vicinity of -8 to -10 R-E, possibly associated wi th magnetosonic energy focusing, acts as a trigger for substorm initiation. It is the element that connects the ionospheric to magnetospheric substorm . Particularly impressive is the timing of the chain of events and indices observed on the ground and their proxies computed in the simulation. This s imulation is complemented by a "theoretician's" simulation, a step function transition of the IMF from northward to southward, which clarifies the phy sics of energy penetration into the magnetosphere and demonstrates Poynting flux focusing in the near earth tail. The second event - January 10-11, 19 97 - was driven by the impact of a magnetic cloud in the magnetosphere. It induced major disturbances in the magnetosphere and the groundand resulted in the loss of a geosynchronous ATT satellite. It is a simulation "tour de force" acid used continuous upstream data over 36 hours as input. The resul ts provide a graphical and fascinating view of the global magnetospheric an d tail response to a magnetic cloud impinging upstream, illustrate the impo rtance of dynamics and indicate that pressure impulses play a key role in p roviding the coherence required for substorms. (C) 1998 Elsevier Science Lt d. All rights reserved.