Cluster/Double Star Observations of Substorm Dipolarization

Simultaneous observations of the inner magnetosphere and the mid tail are essential in substorm studies because of the initial local onset and the subsequent global expansion of the disturbance. Particularly, how these two key regions are linked in terms of fast flow and magnetic field dipolarizations are yet to be determined to understand the mechanism of substorm development. In this paper, we present the first analysis of substorm dipolarizations observed by Cluster and Double Star during the 2004 summer, when successive radial conjunctions between 10-12 Re and 16-19 Re were realized. By using multi-point analysis techniques, the direction and speed of the propagation is determined using Cluster and is then compared with the global propagation of the disturbances with Double Star. Recurrence of these dipolarization events enabled us to draw a more concise picture of the substorm development.