A recent trend in substorm research is the time history approach, motivated by the NASA mission THEMIS (Time History of Events and Macroscale Interactions during Substorms). 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 with current disruption that does not 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.
↧