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Ann. Geophys., 22, 289-302, 2004
www.ann-geophys.net/22/289/2004/
© European Geosciences Union 2004
A study of Pc-5 ULF oscillations
M. K. Hudson1, R. E. Denton1, M. R. Lessard2, E. G. Miftakhova1, and R. R. Anderson3
1Department of Physics and Astronomy, Dartmouth College, Hanover, NH 03755, USA
2Thayer School of Engineering, Dartmouth College, Hanover, NH 03755, USA
3Department of Physics and Astronomy, University of Iowa, Iowa City, IA 52245, USA
Abstract. A study of Pc-5 magnetic pulsations using data from the Combined
Release and Radiation Effects Satellite (CRRES) was carried out.
Three-component dynamic magnetic field spectrograms
have been used to survey ULF pulsation activity for the approximate
fourteen month lifetime of CRRES. Two-hour panels
of dynamic spectra were examined to find events which fall into
two basic categories: 1) toroidal modes (fundamental and harmonic
resonances) and 2) poloidal modes, which include compressional
oscillations. The occurence rates were determined as a function of
L value and local time. The main result is a comparable
probability of occurence of toroidal mode oscillations on the dawn
and dusk sides of the magnetosphere inside geosynchronous orbit,
while poloidal mode oscillations occur predominantly along the
dusk side, consistent with high azimuthal mode number excitation
by ring current ions.
Pc-5 pulsations following Storm Sudden Commencements (SSCs) were
examined separately. The spatial distribution of modes for the
SSC events was consistent with the statistical study for the
lifetime of CRRES. The toroidal fundamental (and harmonic)
resonances are the dominant mode seen on the dawn-side of the
magnetosphere following SSCs. Power is mixed in all three
components. In the 21 dusk side SSC events there were only a few
examples of purely compressional (two) or radial (one) power in
the CRRES study, a few more examples of purely toroidal modes
(six), with all three components predominant in about half (ten)
of the events.
Key words. Magnetospheric physics (MHD waves and instabilities;
magnetospheric configuration and dynamics) –
Space plasma physics (waves and instabilities)
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