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Volume 19, issue 10/12 | Copyright

Special issue: CLUSTER

Ann. Geophys., 19, 1613-1640, 2001
https://doi.org/10.5194/angeo-19-1613-2001
© Author(s) 2001. This work is distributed under
the Creative Commons Attribution 3.0 License.

  30 Sep 2001

30 Sep 2001

Coordinated Cluster and ground-based instrument observations of transient changes in the magnetopause boundary layer during an interval of predominantly northward IMF: relation to reconnection pulses and FTE signatures

M. Lockwood2,1, A. Fazakerley3, H. Opgenoorth4, J. Moen6,5, A. P. van Eyken7, M. Dunlop14, J.-M. Bosqued8, G. Lu9, C. Cully17, P. Eglitis4, I. W. McCrea1, M. A. Hapgood1, M. N. Wild1, R. Stamper1, W. Denig15, M. Taylor3, J. A. Wild10, G. Provan10, O. Amm11, K. Kauristie11, T. Pulkkinen11, A. Strømme12, P. Prikryl13, F. Pitout4, A. Balogh14, H. Rème8, R. Behlke4, T. Hansen12, R. Greenwald16, H. Frey21, S. K. Morley2, D. Alcaydé8, P.-L. Blelly8, E. Donovan17, M. Engebretson18, M. Lester10, J. Watermann19, and M. F. Marcucci20 M. Lockwood et al.
  • 1Solar Terrestrial Physics Division, Department of Space Science and Technology, Rutherford Appleton Laboratory, Oxfordshire, UK
  • 2Department of Physics and Astronomy, Southampton University, Southampton, UK
  • 3Mullard Space Science Laboratory, Holmbury St. Mary, Surrey, UK
  • 4IRF, Swedish Institute of Space Physics, Uppsala Division, Sweden
  • 5Department of Physics, University of Oslo, Blindern, Oslo, Norway
  • 6Also at Arctic Geophysics, University Courses on Svalbard, Longyearbyen, Norway
  • 7EISCAT Scientific Association, Longyearbyen, Svalbard, Norway
  • 8CESR, Centre d’Etude Spatiale des Rayonnements, Toulouse, France
  • 9High Altitude Observatory, National Center for Atmospheric Research, Boulder, Colorado, USA
  • 10Department of Physics and Astonomy, Leicester University, Leicester, UK
  • 11Finnish Meteorological Institute, Helsinki, Finland
  • 12University of Tromsø, Tromsø, Norway
  • 13Communications Research Centre, Ottawa, Ontario, Canada
  • 14Blackett Laboratory, Imperial College, London, UK
  • 15AFRL, Hanscom AFB, Cambridge, MA, USA
  • 16Remote Sensing Group, Applied Physics Laboratory, John Hopkins University, Laurel, MD, USA
  • 17University of Calgary, Calgary, Canada
  • 18Department of Physics, Augsburg College, Minneapolis, MN, USA
  • 19Danish Meteorological Institute, Kobenhavn, Denmark
  • 20Istituto di Fisica dello Spazio Interplanetario – CNR, Rome, Italy
  • 21Space Science Centre, Berkeley, USA

Abstract. We study a series of transient entries into the low-latitude boundary layer (LLBL) of all four Cluster spacecraft during an outbound pass through the mid-afternoon magnetopause ( [ XGSM, YGSM, ZGSM ] ≈ [ 2, 7, 9 ] RE). The events take place during an interval of northward IMF, as seen in the data from the ACE satellite and lagged by a propagation delay of 75 min that is welldefined by two separate studies: (1) the magnetospheric variations prior to the northward turning (Lockwood et al., 2001, this issue) and (2) the field clock angle seen by Cluster after it had emerged into the magnetosheath (Opgenoorth et al., 2001, this issue). With an additional lag of 16.5 min, the transient LLBL events correlate well with swings of the IMF clock angle (in GSM) to near 90°. Most of this additional lag is explained by ground-based observations, which reveal signatures of transient reconnection in the pre-noon sector that then take 10–15 min to propagate eastward to 15 MLT, where they are observed by Cluster. The eastward phase speed of these signatures agrees very well with the motion deduced by the cross-correlation of the signatures seen on the four Cluster spacecraft. The evidence that these events are reconnection pulses includes: transient erosion of the noon 630 nm (cusp/cleft) aurora to lower latitudes; transient and travelling enhancements of the flow into the polar cap, imaged by the AMIE technique; and poleward-moving events moving into the polar cap, seen by the EISCAT Svalbard Radar (ESR). A pass of the DMSP-F15 satellite reveals that the open field lines near noon have been opened for some time: the more recently opened field lines were found closer to dusk where the flow transient and the poleward-moving event intersected the satellite pass. The events at Cluster have ion and electron characteristics predicted and observed by Lockwood and Hapgood (1998) for a Flux Transfer Event (FTE), with allowance for magnetospheric ion reflection at Alfvénic disturbances in the magnetopause reconnection layer. Like FTEs, the events are about 1 RE in their direction of motion and show a rise in the magnetic field strength, but unlike FTEs, in general, they show no pressure excess in their core and hence, no characteristic bipolar signature in the boundary-normal component. However, most of the events were observed when the magnetic field was southward, i.e. on the edge of the interior magnetic cusp, or when the field was parallel to the magnetic equatorial plane. Only when the satellite begins to emerge from the exterior boundary (when the field was northward), do the events start to show a pressure excess in their core and the consequent bipolar signature. We identify the events as the first observations of FTEs at middle altitudes.

Key words. Magnetospheric physics (magnetopause, cusp and boundary layers; magnetosphere-ionosphere interactions; solar wind-magnetosphere interactions)

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