WESTWARD MOVING DYNAMIC SUBSTORM FEATURES OBSERVED WITH THE IMAGE MAGNETOMETER NETWORK AND OTHER GROUND-BASED INSTRUMENTS

We present the ground signatures of dynamic substorm features with par ticular emphasis on the event interpretation capabilities provided by the IMAGE magnetometer network. This array covers the high latitudes f rom the sub-auroral to the cusp/cleft region. An isolated substorm on 11 Oct. 1993 during the late evening hours exhibited many of well-know n 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 elect rojet. Besides the magnetic field measurements, there were auroral obs ervations and plasma flow and conductivity measurements obtained by EI SCAT. The data of all three sets of instruments are consistent with th e notion of upward field-aligned currents associated with the moving a uroral patches. A detailed analysis of the electrodynamic parameters i n the ionosphere, however, reveals that they do not agree with the exp ectations resulting from commonly used simplifying approximations. For example, the westward moving auroral streamers which are associated w ith field-aligned current filaments, are not collocated with the centr es of equivalent current vortices. Furthermore, there is a clear discr epancy between the measured plasma flow direction and the obtained equ ivalent current direction. All this suggests that steep conductivity g radients are associated with the transient auroral forms. Also self-in duction effects in the ionosphere may play a role for the orientation of the plasma flows. This study stresses the importance of multi-instr ument observation for a reliable interpretation of dynamic auroral pro cesses.