Electrodynamics of the poleward auroral border observed by Polar during a substorm on April 22, 1998

Observations from Polar during a substorm on April 22, 1998, are used to sp ecify electrodynamic characteristics of the high-latitude auroral boundary on the nightside. Polar was moving equatorward near invariant latitude 72 d egrees, 2305 magnetic local time as it crossed the auroral boundary near th e end of the substorm's expansion phase. This boundary was marked by severe east-west plasma flow shears, a reversal of the in-track electric field co mponent, and multiple field-aligned currents. Harmonizing ground measuremen ts with auroral images and in situ particle and field data from Polar revea ls five electrodynamic features of the boundary. (1) A 20-min delay occurre d between substorm onset and when the total magnetic flux in the polar cap began to decrease. This represents the time that elapsed before reconnectio n of open lobe flux began along a near-Earth X-line. (2) The reconnection e lectric field at the ionospheric projection of the X-line ranged between 20 and 70 mV m(-1) Reconnection was intermittent, turning on and off at diffe rent locations. (3) Electric and magnetic field structures observed by Pola r suggest that Alfven waves propagating along the auroral boundary carried a double-layer current. Downward Poynting flux was observed at the poleward auroral boundary associated with these currents. (4) Magnetic and electric field oscillations with periods of similar to 90 s were detected on open f ield lines beginning similar to4 min before Polar entered the auroral oval. Oscillations with similar frequencies were observed both on the ground nea r Polar's magnetic footprint and at geosynchronous orbit. This indicates th at the oscillations represent a large-scale phenomenon occurring over a lar ge portion of the nightside magnetosphere. Coupling on open field lines der ives from fringing fields associated with ionospheric closure of DP 1 curre nts. (5) Upward flowing hydrogen and oxygen ions were detected at and equat orward of the auroral boundary. Perpendicularly accelerated O+ ions detecte d in the immediate vicinity of the boundary can be explained by direct acce leration by the ambient electric field perpendicular to the local magnetic field. Equatorward of the boundary, O+ distributions were typical of ion co nics.