Enhanced transport across entire length of plasma sheet boundary field lines leading to substorm onset

[1] Recently, images from the THEMIS all‐sky imager array were used to suggest that longitudinally localized enhanced plasma transport across the nightside open‐closed boundary (separatrix) ∼5.5 min before substorm onset leads to intrusion of new plasma toward the near‐Earth plasma sheet and to substorm onset via near‐Earth instability. If such reconnection‐related flow enhancements precede substorm onsets, they should be seen along the entire length of separatrix field lines within the appropriate longitudinal region as perpendicular flow before onset. Evidence for such flows has been previously reported for four events over a limited radial extent. Based on 13 substorm events with at least one of the THEMIS spacecraft near the outer boundary of the plasma sheet in the premidnight region and simultaneous auroral images, we find here that enhanced flows toward the plasma sheet center are often seen (11 of 13 events) in the premidnight sector a few minutes prior to onset from X ∼ −11 to −26 RE. Previous radar observations within the ionosphere have also shown that such enhanced flows bringing new plasma into the auroral zone from polar cap field lines prior to onset are common. Assuming that the outer boundary of the plasma sheet approximately demarcates the open‐closed field line boundary, these results indicate that reconnection‐related flow enhancements do indeed precede substorm onset along the entire radial extent of the plasma sheet outer boundary. Consistent with longitudinal localization, enhanced flows toward the plasma sheet center were not observed on all spacecraft for five events where there were more than one spacecraft near the plasma sheet boundary.