Substorm Current Wedge as a Combined Effect of Wedgelets

Abstract:

Understanding how the substorm current wedge (SCW) is formed is crucial to solving the substorm mystery. One recent idea on the SCW formation is the “wedgelets” picture, which proposes that dipolarizing flux bundles (DFBs) are the building elements of an SCW. (A DFB is a ~1-3 RE wide flux tube with stronger magnetic field than the ambient plasma in the magnetotail; its leading edge is known as a “dipolarization front”, or “reconnection front”, the product of near-Earth reconnection). Although each DFB carries field-aligned currents (FACs) in similar configuration to an SCW, it is unclear how the DFBs combine to become the large-scale (several magnetic local times wide) region-1-sense (towards Earth at the dawn sector of the magnetotail and away from Earth at the dusk sector) FACs of the SCW. To answer this question, we investigate the FACs of DFBs statistically using THEMIS data. Our results suggest that at the dawn (dusk) sector of the magnetotail, a DFB has more FAC towards (away from) Earth than away from (towards) Earth, so that the net FAC is towards (away from) Earth. The combined effect of many DFBs is therefore the same as the large-scale region-1-sense SCW, supporting the idea that “wedgelets” comprise the large scale substorm current wedge.