Reconciliation of the substorm onset determined on the ground and at the Polar spacecraft

An isolated substorm on Oct. 17, 1997 during a close conjunction of the Pol ar spacecraft and the ground-based MIRACLE network is studied in detail. We identify signatures of substorm onset in the plasma sheet midway between t he ionosphere and the equatorial plasma sheet, determine their timing relat ive 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 a uroral precipitation, energization of plasma sheet electrons at Polar, and strong magnetic field variations perpendicular to the ambient field. Such a ccurate timing coincidence is consistent with the Alfven transit time betwe en Polar and the ionosphere. Furthermore, the timing of other field and par ticle signatures at Polar showed clear deviations from the onset time (+/- 2 min). This suggests that the sequence of these signatures around the onse t time can be used to validate the signatures predicted by various substorm onset models.