Geoscience and Engineering Center, SRI International, 333 Ravenswood Avenue, Menlo Park, CA 94025

High-latitude observations on 18 January 1984 during the first GISMOS campaign (Global Ionospheric Simultaneous Measurements of Substorms) are presented. An intense substorm was initiated after a long period of quiescent geomagnetic activity when the interplanetary magnetic field decreased gradually over a 2-h period from +5 to -8 nT. How the auroral precipitation boundary, the energy flux, and the convection respond to this change from quiet to active conditions is investigated. The results concerning the auroral particle precipitation can be summarized as follows: P, the power dissipated in one hemisphere from electron and proton precipitation does not appear to be directly driven by the solar wind e function. The polar cap area increases as -B_Z increases. The cross-polar-cap potential drop estimated by the variation in open magnetic flux agrees well with the estimate based on an empirical function. Most of the increase in P is due to the increase in energy flux in the midnight sector and to the increase in latitudinal width of precipitation in the post-midnight sector. The results concerning the high-latitude convection pattern can be summarized as follows: the convection intensifies within minutes after B_Z starts to decrease. The convection reversal latitude moves poleward while B_Z is decreasing but still positive. It moves equatorward when B_Z becomes negative and while B_Z continues to decrease. In the early-morning sector, there is a considerable plasma flow across the convection reversal, from the region of sunward flow towards the region of anti-sunward flow. This finding contradicts the basic assumption inherent in some theoretical models.

Ann. Geophys., 5A, (6), 519-526, July 1987