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Annales Geophysicae An interactive open-access journal of the European Geosciences Union
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Volume 27, issue 3
Ann. Geophys., 27, 1251–1266, 2009
© Author(s) 2009. This work is distributed under
the Creative Commons Attribution 3.0 License.
Ann. Geophys., 27, 1251–1266, 2009
© Author(s) 2009. This work is distributed under
the Creative Commons Attribution 3.0 License.

  12 Mar 2009

12 Mar 2009

Shape, size, velocity and field-aligned currents of dayside plasma injections: a multi-altitude study

A. Marchaudon1, J.-C. Cerisier2, M. W. Dunlop3, F. Pitout4, J.-M. Bosqued5, and A. N. Fazakerley6 A. Marchaudon et al.
  • 1Laboratoire de Physique et Chimie de l'Environnement et de l'Espace, CNRS et Université d'Orléans, 3A avenue de la Recherche Scientifique, 45071 Orléans Cedex 2, France
  • 2Laboratoire de Physique des Plasmas, Ecole Polytechnique, CNRS et Université Pierre et Marie Curie-Paris 6, 4 avenue de Neptune, 94107 Saint-Maur-des-Fossés Cedex, France
  • 3Rutherford-Appleton Laboratory, Chilton, Didcot, Oxon, OX11 0QX, UK
  • 4Laboratoire de Planétologie de Grenoble, CNRS et Université de Grenoble 1, BP 53, 38041 Grenoble Cedex 9, France
  • 5Centre d'Etude Spatiale des Rayonnements, CNRS et Université de Toulouse 3, 9 avenue du Colonel Roche, 31028 Toulouse Cedex 4, France
  • 6Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking, RH5 6NT, Surrey, UK

Abstract. On 20 February 2005, Cluster in the outer magnetosphere and Double Star-2 (TC-2) at mid-altitude are situated in the vicinity of the northern cusp/mantle, with Cluster moving sunward and TC-2 anti-sunward. Their magnetic footprints come very close together at about 15:28 UT, over the common field-of-view of SuperDARN radars. Thanks to this conjunction, we determine the velocity, the transverse sizes, perpendicular and parallel to this velocity, and the shape of three magnetic flux tubes of magnetosheath plasma injection. The velocity of the structures determined from the Cluster four-spacecraft timing analysis is almost purely antisunward, in contrast with the antisunward and duskward convection velocity inside the flux tubes. The transverse sizes are defined from the Cluster-TC-2 separation perpendicular to the magnetic field, and from the time spent by a Cluster spacecraft in one structure; they are comprised between 0.6 and 2 RE in agreement with previous studies. Finally, using a comparison between the eigenvectors deduced from a variance analysis of the magnetic perturbation at the four Cluster and at TC-2, we show that the upstream side of the injection flux tubes is magnetically well defined, with even a concave front for the third one giving a bean-like shape, whereas the downstream side is far more turbulent. We also realise the first quantitative comparison between field-aligned currents at Cluster calculated with the curlometer technique and with the single-spacecraft method, assuming infinite parallel current sheets and taking into account the velocity of the injection flux tubes. The results agree nicely, confirming the validity of both methods. Finally, we compare the field-aligned current distribution of the three injection flux tubes at the altitudes of Cluster and TC-2. Both profiles are fairly similar, with mainly a pair of opposite field-aligned currents, upward at low-latitude and downward at high-latitude. In terms of intensity, the field-aligned currents at Cluster are two to three times less intense than at TC-2 for the first two flux tubes, in agreement with magnetic field line convergence. For the third flux tube, the intensity is equal, which is explained by the fact that TC-2 crosses the tube on its edge. Finally, the analysis of the ion and electron moments at Cluster shows that the field-aligned currents result from a small difference between upward ion and electron fluxes.

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