The Substorm Onset Location Controversy

Recent development of popular substorm onset models was driven by the apparent controversy between ground onset mapping in the near-Earth tail and statistics of tailward flow observations (reconnection signatures), pointing to ∼ -25 R E downtail. However, several comprehensive multi-point event studies, taking into account backward tracing of flows, place plasmoid origin closer to the near-Earth location, in better consistency with auroral onset observations. Accuracy of such remote onset positioning is about a couple of Earth radii. We further discuss possibilities of reconciliation of different onset scenarios and future critical experiments.