Abstract. We present the ground signatures of dynamic substorm features with particular emphasis on the event interpretation capabilities provided by the IMAGE magnetometer network. This array covers the high latitudes from the sub-auroral to the cusp/cleft region. An isolated substorm on 11 Oct. 1993 during the late evening hours exhibited many of well-known features such as the Harang discontinuity, westward travelling surge and poleward leap, but also discrete auroral forms, known as auroral streamers, appeared propagating westward along the centre of the electrojet. Besides the magnetic ®eld measurements, there were auroral observations and plasma¯ow and conductivity measurements obtained by EISCAT. The data of all three sets of instruments are consistent with the notion of upward ®eld-aligned currents associated with the moving auroral patches. A detailed analysis of the electrodynamic parameters in the ionosphere, however, reveals that they do not agree with the expectations resulting from commonly used simplifying approximations. For example, the westward moving auroral streamers which are associated with ®eld-aligned current ®laments, are not collocated with the centres of equivalent current vortices. Furthermore, there is a clear discrepancy between the measured plasma¯ow direction and the obtained equivalent current direction. All this suggests that steep conductivity gradients are associated with the transient auroral forms. Also self-induction eects in the ionosphere may play a role for the orientation of the plasma¯ows. This study stresses the importance of multi-instrument observation for a reliable interpretation of dynamic auroral processes.

Abstract. Using the method of characteristics to invert ground-based data of the ground magnetic ®eld disturbance and of the ionospheric electric ®eld, we obtain spatial distributions of ionospheric conductances, currents, and ®eld-aligned currents (FACs) associated with a north-south auroral form that drifts westwards over northern Scandinavia around 2200 UT on December 2, 1977. This auroral form is one in a sequence of such north-south structures observed by all-sky cameras, and appears 14 min after the last of several breakups during that extremely disturbed night. Our analysis shows that the ionospheric Hall conductance reaches values above 200 S in the center of the form, and upward¯owing FACs of up to 25 lA/m 2 are concentrated near its westward and equatorward edge. The strong upward owing FACs are fed by an area of more distributed, but still very strong downward-¯owing FACs northeastward of the auroral form. In contrast to the conductances, the electric ®eld is only slightly a ected by the passage of the form. We point out similarities and di erences of our observations and results to previously reported observations and models of`auroral ®ngers', north-south aurora', and`auroral streamers' which are suggested to be ionospheric manifestations of bursty bulk¯ows in the plasma sheet.

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Abstract. Using the method of characteristics to invert ground-based data of the ground magnetic field disturbance and of the ionospheric electric field, we obtain spatial distributions of ionospheric conductances, currents, and field-aligned currents (FACs) associated with a north-south auroral form that drifts westwards over northern Scandinavia around 2200 UT on December 2, 1977. This auroral form is one in a sequence of such north-south structures observed by all-sky cameras, and appears 14 min after the last of several breakups during that extremely disturbed night. Our analysis shows that the ionospheric Hall conductance reaches values above 200 S in the center of the form, and upward flowing FACs of up to 25 µA/m2 are concentrated near its westward and equatorward edge. The strong upward flowing FACs are fed by an area of more distributed, but still very strong downward-flowing FACs northeastward of the auroral form. In contrast to the conductances, the electric field is only slightly affected by the passage of the form. We point out similarities and differences of our observations and results to previously reported observations and models of 'auroral fingers', 'north-south aurora', and 'auroral streamers' which are suggested to be ionospheric manifestations of bursty bulk flows in the plasma sheet.Key words. Ionosphere (auroral ionosphere; electric fields and currents) · Magnetospheric physics (magnetosphere · ionosphere interactions)

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