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Annales Geophysicae An interactive open-access journal of the European Geosciences Union
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Volume 26, issue 9
Ann. Geophys., 26, 2845-2852, 2008
https://doi.org/10.5194/angeo-26-2845-2008
© Author(s) 2008. This work is distributed under
the Creative Commons Attribution 3.0 License.

Special issue: STAMMS: Spatio-Temporal Analysis and Multipoint Measurements...

Ann. Geophys., 26, 2845-2852, 2008
https://doi.org/10.5194/angeo-26-2845-2008
© Author(s) 2008. This work is distributed under
the Creative Commons Attribution 3.0 License.

  22 Sep 2008

22 Sep 2008

Cluster observations of near-Earth magnetospheric lobe plasma densities – a statistical study

K. R. Svenes1, B. Lybekk2, A. Pedersen2, and S. Haaland3,4 K. R. Svenes et al.
  • 1Norwegian Defence Research Establishment, P.O. Box 25, 2027 Kjeller, Norway
  • 2University of Oslo, Department of Physics, Norway
  • 3University of Bergen, Department of Physics, Norway
  • 4Max-Planck Institute for Solar System Research, Lindau, Germany

Abstract. The Cluster-mission has enabled a study of the near-Earth magnetospheric lobes throughout the waning part of solar cycle 23. During the first seven years of the mission the satellites crossed this region of space regularly from about July to October. We have obtained new and more accurate plasma densities in this region based on spacecraft potential measurements from the EFW-instrument. The plasma density measurements are found by converting the potential measurements using a functional relationship between these two parameters. Our observations have shown that throughout this period a full two thirds of the measurements were contained in the range 0.007–0.092 cm−3 irrespective of solar wind conditions or geomagnetic activity. In fact, the most probable density encountered was 0.047 cm−3, staying roughly constant throughout the entire observation period. The plasma population in this region seems to reflect an equilibrium situation in which the density is independent of the solar wind condition or geomagnetic activity. However, the high density tail of the population (ne>0.2 cm−3) seemed to decrease with the waning solar cycle. This points to a source region influenced by the diminishing solar UV/EUV-intensity. Noting that the quiet time polar wind has just such a development and that it is magnetically coupled to the lobes, it seems likely to assume that this is a prominent source for the lobe plasma.

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