Four spacecraft study of dipolarization-like signatures at the lobe/plasma sheet interface

Observations from the four Cluster spacecraft are utilized for a study of magnetic dipolarization-like events. The focus is on measurements obtained at the lobe/plasma sheet interface in the midnight sector at a distance of about 15 Re downtail. From this position we observe the dynamics of the reconfiguration of the tail magnetosphere associated with the substorm expansion phase. Empirical observations at Cluster related to these events include bursty bulk flows, dipolarization-like magnetic field changes, a sudden reappearance of the plasma sheet with a significant increase in particle energy relative to the prior situation as well as strong E-field fluctuations. These events are transient in nature and often appear in multiple trains. The time periods in between the events, typically a few hours, are characterized by a gradual decrease of plasma energy and stretching of the magnetic field at the Cluster location. We plan to discuss the relationship between these transient observations and substorm onset timing. The timing among the signatures observed at Cluster varies between events. This fact will be utilized in the discussion of the physics of these events. In addition, multi-spacecraft techniques will be used to determine the direction of boundary motions associated with the events.