Timing of magnetic reconnection initiation during a global magnetospheric substorm onset

We have used a unique constellation of Earth‐orbiting spacecraft and ground‐based measurements in order to study a relatively isolated magnetospheric substorm event on August 27, 2001. Global ultraviolet images of the northern auroral region established the substorm expansion phase onset at 0408:19 (±1 min) UT. Concurrent measurements from the GOES‐8, POLAR, LANL, and CLUSTER spacecraft allow us to construct a timeline which is consistent with …