Injection region propagation outside of geosynchronous orbit

Using radial alignments of the Polar and Geotail satellites with the Los Alamos National Laboratory (LANL) fleet of geosynchronous observations, we investigate the radial propagation of the dispersionless substorm injection region outside 6.6 RE. We compare the delay between injection onset observed at geosynchronous orbit and a second spacecraft in the same meridian (within 1 hr of Magnetic Local Time (MLT)) but at a different radial distance. Our results are consistent with earlier studies showing predominantly Earthward propagation of the substorm injection region at or near geosynchronous orbit. However, observations with spacecraft located further down tail (R > ∼9 RE) conclusively show that tailward propagation of the dispersionless injection region must also occur. A statistical study of events using 10 years of Polar, Geotail, and LANL observations shows that dispersionsless injections are most likely to initiate at radial distances of 6.6–9 RE. Injections typically start at this location and expand radially inward toward geosynchronous orbit and outward into the midtail central plasma sheet. Implications of these results on injection region models are discussed.