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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, 3161–3172, 2006
https://doi.org/10.5194/angeo-24-3161-2006
© Author(s) 2006. This work is distributed under
the Creative Commons Attribution 3.0 License.
Ann. Geophys., 24, 3161–3172, 2006
https://doi.org/10.5194/angeo-24-3161-2006
© Author(s) 2006. This work is distributed under
the Creative Commons Attribution 3.0 License.

  22 Nov 2006

22 Nov 2006

On the excitation of ULF waves by solar wind pressure enhancements

P. T. I. Eriksson1, L. G. Blomberg1, S. Schaefer2, and K.-H. Glassmeier2 P. T. I. Eriksson et al.
  • 1Space and plasma physics, School of Electrical Engineering, Royal Institute of Technology, Stockholm, Sweden
  • 2Institute for Geophysics and Extraterrestrial Physics, Technical University of Braunschweig, Braunschweig, Germany

Abstract. We study the onset and development of an ultra low frequency (ULF) pulsation excited by a storm sudden commencement. On 30 August 2001, 14:10 UT, the Cluster spacecraft are located in the dayside magnetosphere and observe the excitation of a ULF pulsation by a threefold enhancement in the solar wind dynamic pressure. Two different harmonics are observed by Cluster, one at 6.8 mHz and another at 27 mHz. We observe a compressional wave and the development of a toroidal and poloidal standing wave mode. The toroidal mode is observed over a narrow range of L-shells whereas the poloidal mode is observed to have a much larger radial extent. By looking at the phase difference between the electric and magnetic fields we see that for the first two wave periods both the poloidal and toroidal mode are travelling waves and then suddenly change into standing waves. We estimate the azimuthal wave number for the 6.8 mHz to be m=10±3. For the 27 mHz wave, m seems to be several times larger and we discuss the implications of this. We conclude that the enhancement in solar wind pressure excites eigenmodes of the geomagnetic cavity/waveguide that propagate tailward and that these eigenmodes in turn couple to toroidal and poloidal mode waves. Thus our observations give firm support to the magnetospheric waveguide theory.

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