Multispacecraft observations are analyzed to examine the dynamics of thin current sheet evolution during the early phases of a substorm during the period 0630--0800 UT on December 10, 1996. The ground signatures showed two activations, one latitudinally localized at 0731 UT and another, larger expansion around 0800 UT. A local model consisting of two current sheets (a thin current sheet and a thicker plasma sheet) was developed utilizing two-spacecraft magnetic field measurements within the magnetotail. Extension of the thin current sheet model to the inner magnetotail shows that during most of the growth phase, the thin current sheet intensified simultaneously and at the same rate in the mid-magnetotail (~30 RE) and near geosynchronous orbit. These results show that a thin current sheet extended from the near-geostationary region to the midtail, and that the entire current sheet responded to the driving in a coherent way. Furthermore, timings suggest that the onset-associated disturbances were observable in the midtail before the ground or geosynchronous signatures. ¿ 1999 American Geophysical Union