An isolated substorm on Oct. 17, 1997 during a close conjunction of the Polar spacecraft and the ground-based MIRACLE network is studied in detail. We identify signatures of substorm onset in the plasma sheet midway between the ionosphere and the equatorial plasma sheet, determine their timing relative to the ground signatures, and discuss their counterparts on the ground and in the equatorial plasma sheet. The substorm onset is determined as the negative bay onset at 2040:42(¿5 sec) UT coinciding with the onset of auroral precipitation, energization of plasma sheet electrons at Polar, and strong magnetic field variations perpendicular to the ambient field. Such accurate timing coincidence is consistent with the Alfv¿n transit time between Polar and the ionosphere. Furthermore, the timing of other field and particle signatures at Polar showed clear deviations from the onset time (¿2 min). This suggests that the sequence of these signatures around the onset time can be used to validate the signatures predicted by various substorm onset models. ¿ 2001 American Geophysical Union