CDAW 7 revisited: Further evidence for the creation of a near‐Earth substorm neutral line

Abstract

The well‐defined CDAW 7 substorm that occurred at 1112 UT on April 24, 1979, has initially been considered one of the most unambiguous examples supporting the near‐Earth neutral line model of magnetospheric substorms. In a recent paper, this point of view has been questioned by Eastman et al. (1988), who state that the magnetic field changes observed on board the ISEE 1 and 2 spacecraft as well as plasma and energetic particle phenomena can be explained simply in terms of spatial movements of a preexisting plasma sheet boundary layer (PSBL) and its associated current sheets across the spacecraft. By contrast, reinvestigation of the ISEE 1 and 2 energetic particle measurements around substorm onset on short time scales demonstrates that the observed flux pattern requires the formation of a particle source earthward of the ISEE spacecraft, well inside the plasma sheet, associated with the substorm onset. The energetic ion pattern observed at ISEE 1 and 2 is consistent with the lobeward progression of field lines being reconnected at a near‐Earth reconnection region. About one minute after the onset of the strong tailward anisotropy of the energetic ions, the lobeward expansion ceases, leading temporarily to an almost stationary state where the outer edge of a separatrix layer is located between ISEE 1 and 2. Remote sensing methods allow us to trace the thinning of the plasma sheet before the 1112 UT substorm onset by determining the contraction speed and the normal vector of the plasma sheet boundary. While both satellites resided within a few ion gyroradii distance from the lobe in the expected PSBL region, strong flows were absent prior to substorm onset, which indicates the temporal nature of the event as opposed to an encounter with a preexisting PSBL containing large flows. At about 3 min after substorm onset, the ISEE spacecraft cross the separatrix and enter an open field line region, consistent with the tailward retreat of a recently formed plasmoid.