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Ann. Geophys., 23, 593-608, 2005
www.ann-geophys.net/23/593/2005/
© European Geosciences Union 2005
Pseudo-field line resonances in ground Pc5 pulsation events
D. V. Sarafopoulos
Department of Electrical and Computer Engineering, Demokritos University of Thrace, Xanthi, Greece
Abstract. In this work we study four representative cases
of Pc5 ground pulsation events with discrete and remarkably stable
frequencies extended at least in a high-latitude range of ~20°; a
feature that erroneously gives the impression for an oscillation mode with
"one resonant field line". Additionally, the presented events show
characteristic changes in polarization sense, for a meridian chain of
stations from the IMAGE array, and maximize their amplitude at or close to
the supposed resonant magnetic field shell, much like the typical FLR.
Nevertheless, they are not authentic FLRs, but pseudo-FLRs, as they are
called. These structures are produced by repetitive and tilted twin-vortex
structures caused by magnetopause surface waves, which are probably imposed
by solar wind pressure waves. The latter is confirmed with in-situ measurements
obtained by the Cluster satellites, as well as the Geotail, Wind, ACE, and
LANL 1994-084 satellites. This research effort is largely based on two
recent works: first, Sarafopoulos (2004a) has observationally established
that a solar wind pressure pulse (stepwise pressure variation) produces a
twin-vortex (single vortex) current system over the ionosphere; second,
Sarafopoulos (2004b) has studied ground events with characteristic
dispersive latitude-dependent structures and showed that these are
associated with twin-vortex ionosphere current systems. In this work, we show
that each pseudo-FLR event is associated with successive and tilted
large-scale twin-vortex current systems corresponding to a magnetopause
surface wave with wavelength 10-20RE. We infer that between an
authentic FLR, which is a spatially localized structure with an extent
0.5RE in the magnetospheric equatorial plane, and the magnetopause surface
wavelength, there is a scale factor of 20-40. A chief observational finding, in
this work, is that there are Pc5 ground pulsation events showing two gradual
and latitude dependent phase-shifts of 180°, at the same time.
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