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Annales Geophysicae An interactive open-access journal of the European Geosciences Union
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Volume 24, issue 11
Ann. Geophys., 24, 3011–3026, 2006
https://doi.org/10.5194/angeo-24-3011-2006
© Author(s) 2006. This work is distributed under
the Creative Commons Attribution 3.0 License.
Ann. Geophys., 24, 3011–3026, 2006
https://doi.org/10.5194/angeo-24-3011-2006
© Author(s) 2006. This work is distributed under
the Creative Commons Attribution 3.0 License.

  22 Nov 2006

22 Nov 2006

Cluster survey of the mid-altitude cusp: 1. size, location, and dynamics

F. Pitout1, C. P. Escoubet2, B. Klecker1, and H. Rème3 F. Pitout et al.
  • 1Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße, 85741 Garching, Germany
  • 2European Space Agency, Keplerlaan 1, 2201 AZ, Noordwijk, The Netherlands
  • 3Centre d'étude Spatiale des Rayonnements, 9, avenue du Colonel Roche, BP 4346, 31028 Toulouse Cedex 4, France

Abstract. We present a statistical study of four years of Cluster crossings of the mid-altitude cusp. In this first part of the study, we start by introducing the method we have used a) to define the cusp properties, b) to sort the interplanetary magnetic field (IMF) conditions or behaviors into classes, c) to determine the proper time delay between the solar wind monitors and Cluster. Out of the 920 passes that we have analyzed, only 261 fulfill our criteria and are considered as cusp crossings. We look at the size, location and dynamics of the mid-altitude cusp under various IMF orientations and solar wind conditions. For southward IMF, Bz rules the latitudinal dynamics, whereas By governs the zonal dynamics, confirming previous works. We show that when |By| is larger than |Bz|, the cusp widens and its location decorrelates from By. We interpret this feature in terms of component reconnection occurring under By-dominated IMF. For northward IMF, we demonstrate that the location of the cusp depends primarily upon the solar wind dynamic pressure and upon the Y-component of the IMF. Also, the multipoint capability of Cluster allows us to conclude that the cusp needs typically more than ~20 min to fully adjust its location and size in response to changes in external conditions, and its speed is correlated to variations in the amplitude of IMF-Bz. Indeed, the velocity in °ILAT/min of the cusp appears to be proportional to the variation in Bz in nT: Vcusp=0.024 ΔBz. Finally, we observe differences in the behavior of the cusp in the two hemispheres. Those differences suggest that the cusp moves and widens more freely in the summer hemisphere.

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