Dipolarization fronts as a signature of transient reconnection in the magnetotail

Dipolarization fronts (DFs), characterized by a strong and steep increase of the tail magnetic field component B-z normal to the neutral plane and preceded by a much less negative dip of B-z, are reported in many observations of bursty bulk flows and substorm activations throughout the whole Earth's magnetotail. It is shown that similar structures appear in full-particle simulations with open boundaries in a transient regime before the steady reconnection in the original Harris current sheet driven out of the equilibrium by the initial X-line perturbation is established. Being secondary reconnection structures propagating with the Alfven speed, DFs are different from the magnetic field pileup regions reported in earlier simulations with closed boundaries. They also differ from the secondary plasmoids with bipolar B-z changes reported in earlier fluid simulations and particle simulations with open boundaries. In spite of their transient nature, DFs are found to form when the force balance is already restored in the system, which justifies their interpretation as a nonlinear stage of the tearing instability developing in two magnetotail-like structures on the left and on the right of the initial central X-line. 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 E x B drift, whereas electrons either follow that drift being completely magnetized or move faster, forming super-Alfvenic jets. In spite of the different motions of electrons and ions, the growth of the front is not accompanied by the corresponding growth of the electrostatic field and the energy dissipation in fronts is dominated by ions.