Current Density Reduction Ahead of Dipolarization Fronts

During their earthward propagation, dipolarization fronts (DFs) interact with the ambient plasma sheet on kinetic scales. The interaction region is important to the front's structure, propagation, and regional closure of the current system. However, the physics in this region, especially of its current system, is poorly understood. We present Time History of Events and Macroscale Interactions during Substorms (THEMIS) observations of the interaction region between DFs and the ambient plasma sheet at x - 12 RE downtail; these observations show that the current density ahead of the DFs is significantly reduced near the neutral plane. We use a two-dimensional particle-in-cell model to simulate the current density reduction ahead of DFs and investigate the physical mechanism that causes it: Ion reflection and acceleration at the front cause positive charge density to build up. The resultant electrostatic field, Ez, is directed away from the neutral plane. The positive cross-tail Ez × Bx drift of electrons (which remain magnetized) does not affect demagnetized ions. This electron-ion decoupling results in a dawnward cross-field current carried by electrons that reduces the cross-tail current ahead of the approaching front.