On the basis of ground magnetometer data from 75 northern hemisphere stations and the ionospheric conductivity distribution estimated from Viking satellite observations of auroral images, various electrodynamic quantities in the polar ionosphere are calculated for the April 1, 1986, Coordinated Data Analysis Workshop (CDAW) 9 substorm. Since the Scandinavia and Russia chains of magnetometers were located in the premidnight-midnight sector during this interval and the estimated conductivity distribution is instantaneous, our data set provides us with a unique opportunity to examine some long-standing problems associated with the substorm expansion onset. Several important findings of this study are summarized as follows: (1) Before the expansion onset of the substorm, intensifications of ionospheric currents or the cross-polar cap potential are very weak in this particular example. Both quantities begin to increase notably only with the initiation of the substorm expansion onset. (2) The intensified westward electrojet flows along the poleward half of the enhanced ionospheric conductivity belt in the midnight sector during the expansion phase, while its equatorward half is occupied by a weak eastward electrojet. (3) The Joule heating rate and the energy input rate of auroral particles are quite comparable preceding the expansion onset.

During the expansion phase of the substorm, however, Joule heating shows a marked intensification, but the latter increases only moderately, indicating that the Joule dissipation is more effective than auroral particle energy input during substorm times. (4) The Hall currents are not completely divergence-free. The corresponding field-aligned currents show highly localized structures during the maximum epoch of the substorm, with the upward current being located in the region of the steepest conductivity gradient on the poleward side of the westward electrojet in the midnight sector. This is indirect evidence that the so-called imperfect Cowling channel is effective behind the westward traveling surge. ¿ American Geophysical Union 1995