On the Contribution of Dipolarizing Flux Bundles to the Substorm Current Wedge and to Flux and Energy Transport

Abstract

Bursty bulk flows and dipolarizing flux bundles within them play an important role in the transport of mass, energy, and magnetic flux in the magnetotail. On the basis of an magnetohydrodynamic simulation of magnetotail reconnection and dipolarization, we investigate the contribution of individual bursts and flux transport events to the buildup of the substorm current wedge, as well as to the earthward transport of magnetic flux and energy. Individual events, defined by increased flow speed (flow bursts), increased cross-tail electric field, or increased (or increasing) magnetic field Bz, are found to be closely related but not identical. Multiple individual magnetic flux transport events collectively contribute to tailward and azimuthal expansion of dipolarization in the inner tail and to an increase of total field-aligned currents toward or away from the ionosphere. In contrast, the current closure across midnight, estimated from the surface currents at the inner (earthward) boundary of the simulation box, was found to remain only a fraction (∼10% or 0.2 MA) of the total Region 1 current into to ionosphere. The simulation showed dipolarization everywhere earthward of the near-Earth x-line, amounting to ∼2.3 ×108 Wb, commensurate with substorm estimates. This can appear at a satellite in various ways, through either classical earthward transport and pileup (outward moving accumulation) or lateral (azimuthal) or tailward (vortical or recoiled) convective motion of dipolarized flux tubes, or a combination of these.