In situ spatiotemporal measurements of the detailed azimuthal substructure of the substorm current wedge

Abstract

The substorm current wedge (SCW) is a fundamental component of geomagnetic substorms. Models tend to describe the SCW as a simple line current flowing into the ionosphere toward dawn and out of the ionosphere toward dusk, linked by a westward electrojet. We use multispacecraft observations from perigee passes of the Cluster 1 and 4 spacecraft during a substorm on 15 January 2010, in conjunction with ground-based observations, to examine the spatial structuring and temporal variability of the SCW. At this time, the spacecraft traveled east-west azimuthally above the auroral region. We show that the SCW has significant azimuthal substructure on scales of 100 km at altitudes of 4000–7000 km. We identify 26 individual current sheets in the Cluster 4 data and 34 individual current sheets in the Cluster 1 data, with Cluster 1 passing through the SCW 120–240 s after Cluster 4 at 1300–2000 km higher altitude. Both spacecraft observed large-scale regions of net upward and downward field-aligned current, consistent with the large-scale characteristics of the SCW, although sheets of oppositely directed currents were observed within both regions. We show that the majority of these current sheets were closely aligned to a north-south direction, in contrast to the expected east-west orientation of the preonset aurora. Comparing our results with observations of the field-aligned current associated with bursty bulk flows (BBFs), we conclude that significant questions remain for the explanation of SCW structuring by BBF-driven “wedgelets.” Our results therefore represent constraints on future modeling and theoretical frameworks on the generation of the SCW.Key PointsThe substorm current wedge (SCW) has significant azimuthal structureCurrent sheets within the SCW are north-south alignedThe substructure of the SCW raises questions for the proposed wedgelet scenario

A physical explanation for the magnetic decrease ahead of dipolarization fronts

“…Both simulations (Birn and Hesse, 2013;Birn et al, 1999) and observations (Yao et al, 2012(Yao et al, , 2014a have shown that the braking of BBFs in the nearEarth magnetotail may lead to current disruption and the formation of the FACs necessary to support an SCW. However, Forsyth et al (2014) showed that the azimuthal structure of the SCW observed at low altitudes was not consistent with previous observations of BBF's FACs. Very recent results suggest that the wedgelet has dawn-dusk asymmetry , which might be a solution for this inconsistency.…”

Overview of Solar Wind–Magnetosphere–Ionosphere–Atmosphere Coupling and the Generation of Magnetospheric Currents

“…Thirdly, substorms are associated with the formation of the substorm current wedge (SCW) with additional field-aligned currents diverting current from the magnetotail into the nightside ionosphere. Clausen et al (2013a) showed that on average the formation of the SCW enhances the nightside portions of the region 1 current system, though it is also known that the substorm FACs can be highly filamentary (e.g., Forsyth et al 2014), on spatial scales much finer than can be resolved with AMPERE, and evolve with time during the expansion phase (e.g., Sergeev et al 2014). Indeed, we found no clear signature of a SCW in the substorm presented in Fig.…”

“…This is inferred from the observed formation of a substorm current wedge (SCW) during expansion phase (Boström 1964;Atkinson 1967;McPherron et al 1973), a pair of upward/downward FACs on either side of the midnight meridian, with westward current closure in the substorm electrojet across the nightside auroral ionosphere. Although it is possible to show that on average the SCW has this simple up/down FAC structure (e.g., Clausen et al 2013a), substorm currents can be highly filamentary (Ohtani et al 1990;Murphy et al 2013;Forsyth et al 2014).…”

Contrasting dynamics of electrons and protons in the near-Earth plasma sheet during dipolarization

“…The magnetohydrodynamics (MHD) simulations performed by showed that the major SCW and pressure buildup occurred when the low-entropy flows are braked and the flows are diverted azimuthally in the near-Earth PS. The simulation results as well as multi-spacecraft observations showed that the braking flows are localized in space, while the associated increase in the B Z magnetic field (the dipolarization) evolves over a wider spatial range, spreading both azimuthally and radially (e.g., Nakamura et al, 2004;Forsyth et al, 2014;Kronberg et al, 2017). This suggests that the SCW consists of multiple small-scale "wedgelets" (e.g., Liu et al, 2013).…”