Magnetic Reconnection Signatures in Near‐Earth Magnetotail Dipolarization Processes: Ion Beams in the Outermost Plasma Sheet

We investigate the dipolarization processes in the near‐Earth magnetotail using magnetic field and plasma observations obtained from the spacecraft Geotail. Dipolarization in the magnetic field configuration represents an intrinsic process that is associated with the onset of magnetospheric substorms. Dipolarization processes inside the plasma sheet up to a radial distance of 15 R E are very similar to those observed at a geosynchronous orbit of 6.6 R E . In the tail lobe–plasma sheet boundary region, unidirectional earthward ion beams with ion energies of approximately more than 10 keV are detected in the outermost plasma sheet field lines, which are expected to be the most recently reconnected field lines at the magnetic reconnection site existing beyond 20 R E in the magnetotail. It is suggested that upward (from the ionosphere toward the magnetotail) field‐aligned currents, which are directly driven by magnetic reconnection, are located in the ion beam layer in the premidnight sector.