LONGITUDINAL STRUCTURE OF SUBSTORM INJECTIONS AT SYNCHRONOUS ORBIT

From multiple-spacecraft measurements it is shown that the synchronous orbit manifestation of a substorm, i. e. , plasma injection and magnetic field reconfiguration to dipolar, has an onset which expands both eastward and westward from a relatively narrow sector near midnight. For low-to-moderate geometric activity the earliest onset sector at synchronous orbit is about 3 hours wide, skewed toward the evening side of midnight. Simultanous westward motion of electrojet intensification is seen in the ground data over a larger longitudinal range than the magnetospheric signatures. Plasma that has already been injected near midnight at synchronous altitude undergoes no further change as a result of the westward and eastward motion of the borders of the plasma. These results are interpreted in terms of an injection front wedge which makes hot plasma accessible to the inner magnetosphere and which spatially expands or propagates with time.