PLASMA AND MAGNETIC-FLUX TRANSPORT ASSOCIATED WITH AURORAL BREAKUPS

Auroral breakups are the first visible sign of a substorm expansion on set. Keying the plasma sheet behavior to onset times of auroral breaku ps may help to identify the substorm onset process. With this goal in mind, we have identified a list of auroral breakups based on global im aging data from the Ultraviolet Imager of the POLAR spacecraft for two favorable viewing periods and have examined the simultaneous plasma m easurements in the tail from GEOTAIL. Synoptic patterns of plasma tran sport and magnetic field changes in the tail surrounding the times of auroral breakups are constructed. The results indicate that the plasma sheet activities associated with auroral breakups are transient and s patially localized. These findings are consistent with the scenario in which expansion phase activities are dominated by localized, transien t disturbances as portrayed by the substorm synthesis model.