Electric Field Structure of Dipolarization Fronts

Dipolarization fronts are sharp boundaries in the magnetic field and plasma which are thought to be produced by transient magnetic reconnection. Dipolarization fronts are frequently observed in the Earth's magnetotail and are often associated with acceleration of energetic electrons. We study the electric field structure of dipolarization fronts observed by Cluster at small separation (200 km) in the magnetotail. Small cluster separation (sub-ion scale) allows us to assume that the structure does not change between the observations by different spacecraft and allows to perform accurate timing analysis. We find large electric fields directed both normal and tangential to the front. In the narrow non-ideal region at the front the normal electric field is balanced by the Hall (JxB/ne) term implying ions are not fully magnetized. The electric filed behind the front (flux pileup region) is balanced by the ion convection. We find that except for the narrow non-ideal region the tangential electric field is zero in the frame of the front implying the front is a tangential boundary with no net flux of plasma across. We compare the Cluster observations with PIC simulation results.