Two-satellite observations of substorm injections at geosynchronous orbit

Energetic particle and plasma measurements from three fairly closely spaced geosynchronous satellites are used to examine the simultaneous behavior of substorm particle injections at spatially separated locations. A total of 43 injection events are identified as dispersionless at two or more satellites. The observations confirm that the ion and electron injection regions are spatially offset in the azimuthal direction, as previously deduced statistically from single-satellite observations and revealed by test-particle trajectories in MHD simulations of geomagnetic tail reconnection. The two-spacecraft observations show further that the injection regions for both ions and electrons expand azimuthally in both the eastward and westward directions over a timescale of several minutes. The observed expansion rates are comparable to expansion rates derived from the test particle/MHD simulations, lending further support to the identification of such reconnection as the ultimate cause of substorm particle injections.