'Dipolarization' fronts as the exhaust fronts from reconnection in the midtail

The leading fronts of fast plasma flows in the magnetotail are often associated with sharp increases of the magnetic field component normal to the current sheet (B_z), conventionally termed dipolarization fronts (DPF). A combination of analysis of Cluster events and PIC reconnection simulations indicates that these propagating fronts are likely due to jet fronts of reconnection exhaust in the midtail (20-30 Re from the Earth) where B_z is nearly zero on the magnetic equatorial plane. The observed peak B_z amplitudes for the DPF events observed at geocentric distances 9 to 18 Re are nearly constant, consistant with the THEMIS observation of a coherent DPF propagating earthward over 10 Re with no appreciable change in the peak B_z value [Runov et al., GRL, 2009], implying that the observed fronts are not generated by disruptions of cross-tail currents which would cause the magnetic field to dipolarize with stronger peak B_z closer to the Earth. The electron and ion distribution functions as well as the suprathermal particles across the front are compared in details with those across the exhaust front of reconnection in PIC simulations. Most of the observed DPF features are consistent with the exhaust jet produced in PIC simulations of Harris-sheet reconnection (no dipole field).