The substorm current wedge in MHD simulations

摘要

Using magnetohydrodynamic (MHD) simulations of magnetotail dynamics, we investigate the build-up and evolution of the substorm current wedge (SCW) and its association with plasma flows from the tail. Three different scenarios are considered: the propagation of magnetic flux ropes of artificially reduced entropy (bubbles), and the formation and propagation of bubbles resulting from magnetic reconnection in the near and far tail. The simulations confirm the important role of the entropy reduction in the earthward penetration of bubbles, as well as in the build-up of field-aligned current signatures attributed to the SCW. Low-entropy flow channels can indeed propagate close to the Earth from the distant tail, as suggested recently. However, this requires substantial entropy reduction, presumably from progression of reconnection into the lobes. The major SCW and pressure build-up occurred when the low-entropy flow channels were braked and the flow diverted azimuthally in the near-Earth region. The flows commonly exhibit multiple narrow channels, separated in space and time, whereas the associated increases in B_z (dipolarization) accumulate over a wider spatial range, spreading both azimuthally and radially. This suggests a picture of the SCW as being composed of multiple smaller "wedgelets," rather than one big wedge. Key Points Substorm current wedge may build up through multiple flow bursts. Strength of wedge currents increases with closer flow approach. Penetration of flows from distant tail requires substantial entropy loss.