Current carriers near dipolarization fronts in the magnetotail: A THEMIS event study

We study current carriers observed within thin current sheets ahead of and during the passage of earthward moving dipolarization fronts in the near-Earth plasma sheet using Time History of Events and Macroscale Interactions During Substorms (THEMIS) multipoint measurements. The fronts are embedded within flow bursts at the initial stage of bursty bulk flow events. Simultaneous north-south and radial separations between probes P3, P4, and P5 and the planar current sheet approximation enable estimation of cross-tail current density in the current sheet ahead of and within the fronts, respectively. The cross-tail current density increase ahead of the fronts, a substorm growth phase signature, is predominantly due to the ion diamagnetic current; at times, however, the electron pressure gradient may contribute up to 60% of the total current density. Note that in this paper we refer to the horizontal (vertical) current sheet as the cross-tail current sheet (current sheet associated with dipolarization fronts). At the dipolarization fronts, the horizontal cross-tail current sheet (with a current density of several nA/m(2)) relaxes, and a vertical current sheet (with a current density of several tens of nA/m(2)), consistent with the thin interface of the front, appears. Thus, the cross-tail current at longitudes adjacent to the flow burst feeds into the dipolarization front's current sheet and may be extended to higher latitudes. The vertical current density also decreases after passage of the front. The pressure gradient of 1-10 keV electrons is a dominant contributor to the current in the dipolarization fronts. In the event studied, probes P1 and P2, which were several Earth radii downtail, reveal a tailward expansion of the current reduction process at a propagation velocity similar to 50 km/s, even as the bulk flow carrying the magnetic flux remains earthward. This study shows how dipolarization fronts and their current systems are building blocks of the large-scale substorm current wedge.