THEMIS observations of current-carriers in the vicinity of sharp dipolarization fronts in the magnetotail

We examine the current carriers in thin current sheets prior to and during dipolarization fronts associated with burst flows in the magnetotail. The events occur in association with magnetospheric substorms. We examine events in 2009, when the North-South separation between THEMIS P5 and P3/P4 probes enables current density estimates assuming a planar cross-tail current approximation. We examine the hypothesis of current sheet thinning in the growth phase of substorms, ahead of the arrival of the dipolarization front. A factor of 2 increase of the average current density is observed, with values of the order of 1-5 nA/m2. Duskward anisotropies of ions in the 10-50 keV energy range are consistent with the ion diamagnetic current supporting the cross tail current under such conditions. With the arrival of the dipolarization front, the horizontal cross tail current sheet relaxes, however the vertical current sheet due to the thin dipolar structure can be very intense. Assuming a stable structure propagating at the flow velocity we determine the spatial evolution of the magnetic field and compute the current density of the approaching current layer to be ~100 nA/m2. Since a thickness of the structure is less than the ion gyroradius, we examine electrons in the energy range of 100-20000 eV as potential current carriers, either due to spatial gradients or due to anisotropy of the electron distribution. We find that electron fluxes are sufficient to support the intense currents in the dipolarization fronts.