We present observations of the Polar spacecraft of magnetospheric substorm signatures in the plasma sheet midway along auroral field lines between the ionosphere and the equatorial plasma sheet. An isolated substorm on October 17, 1997, is studied in detail. The onset time at 2040:42 UT was defined to be almost simultaneous (within 20 sec) on the ground and at Polar altitude (~ 4RE. At Polar, the onset was manifested as strong magnetic field variations and plasma flow bursts with amplitudes exceeding 20 nT and 100 km/s, respectively. Bursts of parallel Poynting flux of ~ 0.5 ergs/cm2 were related to these variations, and they were predominantly directed toward the ionosphere. In addition, a sequence of weak magnetic field variations and flow bursts were observes at Polar ~ 10 min before the onset. Associated with this, a weak westward electrojet was observed on the ground. We discuss the importance of these observations in the physics of substorm onset.