Superposed epoch analysis of magnetotail flux transport during substorms observed by THEMIS

Cumulative magnetic flux transport earthward/tailward of the reconnection site in the plasma sheet or equatorward toward the neutral sheet (Phi) has been shown to be one of the most useful quantities for remotely sensing reconnection onset in the magnetotail. We examine the behavior of Phi during substorms near the onset meridian using superposed epoch analysis of Time History of Events and Macroscale Interactions during Substorms (THEMIS) probe observations at different downtail distances. Observational data come from the THEMIS Substorm Database, assembled under the auspices of the Geospace Environment Modeling (GEM) program (http://www.igpp.ucla.edu/themis/events/). We find that Phi starts to increase a few minutes prior to ground midlatitude Pi2 onset. Although our study cannot monitor regions beyond 30 R-E, the apogee of the most distant probe (P1), enhanced transport tends to begin at 20-30 R-E and moves progressively inward just prior to ground Pi2 onset. Our results are consistent with recent THEMIS case studies showing that reconnection initiates the substorm expansion phase process.