title: 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.
  
  
  CONCLUSIONS
  The 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.
authors.affiliation: Mullard Space Science Laboratory, UCL Dorking, UK.
authors.forename: C
authors.initials: C
authors.lastname: Forsyth
authors.affiliation: Mullard Space Science Laboratory, UCL Dorking, UK.
authors.forename: A N
authors.initials: AN
authors.lastname: Fazakerley
authors.affiliation: Mullard Space Science Laboratory, UCL Dorking, UK.
authors.forename: I J
authors.initials: IJ
authors.lastname: Rae
authors.affiliation: Department of Meteorology, University of Reading Reading, UK.
authors.forename: C E
authors.initials: CE
authors.lastname: J Watt
authors.affiliation: University of Alberta Edmonton, Alberta, Canada.
authors.forename: K
authors.initials: K
authors.lastname: Murphy
authors.affiliation: Lancaster University Lancaster, UK.
authors.forename: J A
authors.initials: JA
authors.lastname: Wild
authors.affiliation: Royal Institute of Technology Stockholm, Sweden.
authors.forename: T
authors.initials: T
authors.lastname: Karlsson
authors.affiliation: Department of Physics and Astronomy, University of Iowa Iowa City, Iowa, USA.
authors.forename: R
authors.initials: R
authors.lastname: Mutel
authors.affiliation: Mullard Space Science Laboratory, UCL Dorking, UK.
authors.forename: C J
authors.initials: CJ
authors.lastname: Owen
authors.affiliation: LASP, University of Colorado Boulder Boulder, Colorado, USA.
authors.forename: R
authors.initials: R
authors.lastname: Ergun
authors.affiliation: ESA/ESTEC Noordwijk, Netherlands.
authors.forename: A
authors.initials: A
authors.lastname: Masson
authors.affiliation: Laboratoire de Physique des Plasmas, Observatoire de Saint Maur Paris, France.
authors.forename: M
authors.initials: M
authors.lastname: Berthomier
authors.affiliation: Department of Physics and Astronomy, University of Calgary Calgary, Alberta, Canada.
authors.forename: E
authors.initials: E
authors.lastname: Donovan
authors.affiliation: Space Sciences Laboratory, University of California Berkeley, California, USA.
authors.forename: H U
authors.initials: HU
authors.lastname: Frey
authors.affiliation: National Space Institute, Technical University of Denmark Lyngby, Denmark.
authors.forename: J
authors.initials: J
authors.lastname: Matzka
authors.affiliation: National Space Institute, Technical University of Denmark Lyngby, Denmark ; GFZ, German Centre for Geosciences Potsdam, Germany.
authors.forename: C
authors.initials: C
authors.lastname: Stolle
authors.affiliation: John Hopkins University Applied Physics Laboratory Laurel, Maryland, USA.
authors.forename: Y
authors.initials: Y
authors.lastname: Zhang
keywords: Aurora; Earth; Field-aligned current; Magnetosphere; Substorm current wedge; Wedgelets