Is the Substorm Current Wedge an Ensemble of Wedgelets?: Revisit to Midlatitude Positive Bays

Abstract:

The goal of the present study is to observationally test the idea that the substorm current wedge (SCW) is an ensemble of wedgelets, meso-scale current systems that correspond to plasma-sheet flow channels. According to this hypothesis, the SML index, SuperMAG equivalent to the AL index, represents a single particular wedgelet at a given time, whereas mid-latitude positive bays are a sum of the remote effects of all wedgelets but with more weighting on ones closer in longitude. However, both event-based and statistical studies of isolated substorms show that (1) mid-latitude N (northward) and E (eastward) ground magnetic variations are highly correlated with SML even far from midnight; (2) the correlation between SML and ground magnetic variations in N and E are organized by the magnetic local time (MLT) of the peak westward electrojet intensity (as identified by SML), and their longitudinal structures are consistent with the conventional SCW model; and (3) the eastward and westward E variations observed at dusk and dawn, respectively, are well correlated. If the SCW is an ensemble of wedgelets, these results would imply that wedgelets with similar intensities are formed side-by-side throughout the SCW and evolve in parallel with each other, which is highly questionable from both physical and morphological points of view. Instead, it is suggested that the SCW is basically a globally coherent system. Although the SCW may evolve from a wedgelet formed at the onset of substorms, wedgelets would not be an essential component for most of the subsequent expansion phase.