Substorm Current Wedge Composition by Wedgelets, Small Pieces of FACs supported by Dipolarization Fronts

Friday, 27 May 2016: 11:15 AM
Jiang Liu1, Vassilis Angelopoulos2, Xiangning Chu1, Xuzhi Zhou1, Chao Yue1, Andrei Runov1 and Zhonghua Yao3, (1)University of California Los Angeles, Los Angeles, CA, United States, (2)University of California Los Angeles, Earth, Planetary, and Space Sciences, Los Angeles, CA, United States, (3)University College London, London, United Kingdom
Abstract:
The substorm current wedge (SCW) is the major field-aligned current (FAC) system of the substorm process, so understanding the SCW is crucial to comprehending the substorm phenomenon. Our recent studies suggest that the SCW could be a collective effect of small-scale FACs carried by dipolarization fronts. Our statistical analysis of THEMIS data showed that the FACs inside the DF layer are directed towards (away from) Earth at the DF’s morning (evening) flank, in a region-1 sense. These FACs are in similar configuration to the SCW’s region-1-sense FACs, although a DF, and thus its FACs, are only ~1 RE wide, much smaller than a SCW which is several-hours-of-MLT wide. The amount of FAC contained by each DF is also much fewer than that of a SCW. We therefore term the FACs of each DF as a wedgelet, a building element of the SCW.

However, it is unclear how these localized wedgelets can collectively form a large-scale SCW. Because each wedgelet contains a pair of oppositely directed FACs, neighboring wedgelets will cancel each other’s FACs. The total FAC of many such wedgelets will still be too small to account for an SCW. To solve this problem, we examine the DF’s FACs with a more comprehensive statistical study and found that they are asymmetric: in the dawn (dusk) sector of the magnetotail, a wedgelet has more FAC toward (away from) the Earth than away from (toward) the Earth, so the net FAC is toward (away from) the Earth. In this way, neighboring wedgelets will not cancel each other’s currents, and it takes only ~10 wedgelets to form a typical substorm current wedge.

The attached figure illustrates our scenario that the collective effect of many asymmetric wedgelets is a substorm current wedge.

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