Dipolarization Front: A Distinctive Feature of the Reconnection Onset in the Magnetotail

Recent particle simulations with open boundaries revealed interesting new effects in collisionless magnetic reconnection, including its intermittent regimes with the formation of the secondary plasmoids in the outflow regions. In this presentation we show that, apart from rather conventional plasmoids forming near the electron diffusion region of the central X-line, there is another group of the secondary reconnection structures that strongly resemble the dipolarization fronts, reported in Geotail, Cluster, and Themis observations of bursty bulk flows and substorm activations in the terrestrial magnetotail. These structures are characterized by a strong and quick increase of the original tail field Bz, normal to the neutral plane, up a half of the lobe field, in contrast to a relatively small and shallow negative dip of Bz in the front precursor, comparable in amplitude to the field Bz prior to the dipolarization onset. Both electrons and ions are magnetized at the front of the dipolarization wave. In contrast, in its trail, ions are unmagnetized and move slower compared to the ExB drift, whereas the magnetized electrons either follow that drift or move even faster, forming super-Alfvenic jets. In spite of drastically different motions of electrons and ions, the formation and growth of the dipolarization front is not accompanied by the corresponding growth of the electrostatic field. This suggests that the electron compressibility effect, stabilizing the ion tearing mode in the tail-like systems with trapped magnetized electrons [Lembege and Pellat, 1982], is strongly attenuated in open systems.