Substorm Processes in the Geomagnetic Tail and their Effect in the Nightside Auroral Zone Ionosphere as Observed by EISCAT [and Discussion]

Current understanding of magnetospheric substorms is reviewed with special emphasis on the relation between space-based and ground-based observations. It is pointed out that the traditional means of monitoring substorms from the ground (by using magnetometers, riometers and auroral observations) give only a selective picture of the whole phenomenon, related to the precipitation of electrons with energies above 1 keV. Measurements by incoherent scatter radar, such as the European incoherent scatter facility (EISCAT), give a more complete and continuous picture. The `neutral line' model of substorms provides a natural, physical basis on which relevant data can be interpreted. In this picture, two sources of flow are anticipated in the nightside auroral zones, one `directly driven' (with a delay of 15-20 min) by the interplanetary magnetic field (IMF) B$_{z}$ component and associated with dayside reconnection, and the other appearing typically an hour after southward turnings of the IMF and associated with rapid tail reconnection during substorms. Evidence for the influence of both sources of flow is found in nightside EISCAT data. These data also reveal that, overall, the nightside ionospheric flow and plasma parameters often vary in a quasi-periodic way with a period of ca. 1 h. In two cases in which concurrent interplanetary data are available it appears that the periodicity is inherent in IMF B$_{z}$, but this is not expressed unmodified in the auroral zone because of the presence of the two sources of flow which depend on IMF B$_{z}$ in different ways.