Revisiting Time History of Events and Macroscale Interactions during Substorms (THEMIS) substorm events implying magnetic reconnection as the substorm trigger

Abstract

[1] A recent trend in substorm research is the time history approach, motivated by the NASA mission Time History of Events and Macroscale Interactions during Substorms (THEMIS). We revisit three THEMIS substorm events recently reported to indicate magnetic reconnection as the substorm trigger. The evidence for the occurrence of magnetic reconnection in these events is critically examined. It is found that they are not compelling cases for magnetic reconnection as the substorm trigger. Instead, equally plausible alternative interpretations, such as current disruption by ballooning and/or cross-field current instabilities, global Alfvenic interaction, and new plasma intrusion, that do not necessarily require a change of magnetic field line topology, could be the substorm trigger. The limitations of using the time history approach to ascertain the physical process for substorm expansion onsets are also discussed.