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Ann. Geophys., 24, 3161-3172, 2006
www.ann-geophys.net/24/3161/2006/
© European Geosciences Union 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
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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