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Volume 21, issue 8
Ann. Geophys., 21, 1737-1751, 2003
https://doi.org/10.5194/angeo-21-1737-2003
© Author(s) 2003. This work is distributed under
the Creative Commons Attribution 3.0 License.
Ann. Geophys., 21, 1737-1751, 2003
https://doi.org/10.5194/angeo-21-1737-2003
© Author(s) 2003. This work is distributed under
the Creative Commons Attribution 3.0 License.

  31 Aug 2003

31 Aug 2003

Mesoscale structure of a morning sector ionospheric shear flow region determined by conjugate Cluster II and MIRACLE ground-based observations

O. Amm1, A. Aikio2, J.-M. Bosqued3, M. Dunlop9,4, A. Fazakerley5, P. Janhunen1, K. Kauristie1, M. Lester6, I. Sillanpää1, M. G. G. T. Taylor5,10, A. Vontrat-Reberac7, K. Mursula2, and M. André8 O. Amm et al.
  • 1Finnish Meteorological Institute, Geophysical Research, P.O. Box 503, FIN-00101 Helsinki, Finland
  • 2University of Oulu, Department of Physical Sciences, P.O. Box 3000, FIN-90014 University of Oulu, Finland
  • 3CESR/CNRS, BP 4346 9, Avenue Colonel Roche, 31028 Toulouse Cedex 4, France
  • 4Blackett Laboratory, Space and Atmospheric Physics, Imperial College of Science, Technology, & Medicine, Prince Consort Rd, South Kensington, London SW7 2BZ, UK
  • 5Mullard Space Science Laboratory, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK
  • 6Radio & Space Plasma Physics Group, Depart. of Physics & Astronomy, University of Leicester, Leicester LE1 7RH, UK
  • 7CETP/UVSQ/IPSL, 10-12 Avenue de l’Europe, F-78140 Velizy, France
  • 8Swedish Institute of Space Physics, Uppsala Division, Box 537, SE-75121 Uppsala, Sweden
  • 9now at: Space Sciences Division, SSTD, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire, OX11 0QX, UK
  • 10now at: Los Alamos National Laboratory, P.O. Box 1663, Mail stop D466, Los Alamos, New Mexico 87545, USA

Abstract. We analyse a conjunction event of the Cluster II spacecraft with the MIRACLE ground-based instrument net-work in northern Fennoscandia on 6 February 2001, between 23:00 and 00:00 UT. Shortly after the spacecraft were located at perigee, the Cluster II satellites’ magnetic footpoints move northwards over Scandinavia and Svalbard, almost perfectly aligned with the central chain of the IMAGE magnetometer network, and cross a morning sector ionospheric shear zone during this passage. In this study we focus on the mesoscale structure of the ionosphere. Ionospheric conductances, true horizontal currents, and field-aligned currents (FAC) are calculated from the ground-based measurements of the IMAGE magnetometers and the STARE coherent scatter radar, using the 1-D method of characteristics. An excellent agreement between these results and the FAC observed by Cluster II is reached after averaging the Cluster measurements to mesoscales, as well as between the location of the convection reversal boundary (CRB), as observed by STARE and by the Cluster II EFW instrument. A sheet of downward FAC is observed in the vicinity of the CRB, which is mainly caused by the positive divergence of the electric field there. This FAC sheet is detached by 0.5°–2° of latitude from a more equatorward downward FAC sheet at the poleward flank of the westward electrojet. This latter FAC sheet, as well as the upward FAC at the equatorward flank of the jet, are mainly caused by meridional gradients in the ionospheric conductances, which reach up to 25 S in the electrojet region, but only ~ 5 S poleward of it, with a minimum at the CRB. Particle measurements show that the major part of the downward FAC is carried by upward flowing electrons, and only a small part by downward flowing ions. The open-closed field line boundary is found to be located 3°–4° poleward of the CRB, implying significant errors if the latter is used as a proxy of the former.

Key words. Ionosphere (electric fields and currents) – Magnetosphere physics (current systems; plasma convection)

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