The physics of substorms as revealed by the ISTP

Spacecraft and ground data combined with multiscale computer models developed by the ISTP program are providing a new and coherent understanding of the 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 first unified picture of a substorm from the magnetospheric and ionospheric viewpoint is currently emerging. Here we use MHD simulations of two particularly well observed and analyzed events to explore the factors that trigger and organize the substorm elements into a coherent entity. The first event - March 9, 1995 - produces clear evidence that impulsive penetration of a large electric field in the vicinity of -8 to -10 R(E), possibly associated with 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 simulation is complemented by a 'theoretician's' simulation, a step function transition of the IMF from northward to southward, which clarifies the physics of energy penetration into the magnetosphere and demonstrates Poynting flux focusing in the near earth tail. The second event - January 10-11, 1997 - 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' and used continuous upstream data over 36 hours as input. The results provide a graphical and fascinating view of the global magnetospheric and tail response to a magnetic cloud impinging upstream, illustrate the importance of dynamics and indicate that pressure impulses play a key role in providing the coherence required for substorms.