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Ann. Geophys., 24, 2719-2733, 2006
www.ann-geophys.net/24/2719/2006/
© European Geosciences Union 2006
Evidence of standing waves during a Pi2 pulsation event observed on Cluster
A. B. Collier1,2, A. R. W. Hughes1, L. G. Blomberg2, and P. R. Sutcliffe3
1School of Physics, University of KwaZulu-Natal, Durban, South Africa
2Dept. of Space and Plasma Physics, School of Electrical Engineering, Royal Institute of Technology, Stockholm, Sweden
3Hermanus Magnetic Observatory, Hermanus, South Africa
Abstract. Observations of Pi2 pulsations at middle and low latitudes have been
explained in terms of cavity mode resonances, whereas transients associated
with field-aligned currents appear to be responsible for the high latitude
Pi2 signature.
Data from Cluster are used to study a Pi2 event observed at 18:09 UTC on
21 January 2003, when three of the satellites were within the plasmasphere
(L=4.7, 4.5 and 4.6) while the fourth was on the plasmapause
or in the plasmatrough (L=6.6). Simultaneous pulsations at
ground observatories and the injection of particles at geosynchronous orbit
corroborate the occurrence of a substorm.
Evidence of a cavity mode resonance is established by considering the phase
relationship between the orthogonal electric and magnetic field components
associated with radial and field-aligned standing waves. The relative phase
between satellites located on either side of the geomagnetic equator
indicates that the field-aligned oscillation is an odd harmonic. Finite
azimuthal Poynting flux suggests that the cavity is effectively open ended
and the azimuthal wave number is estimated as m~13.5.
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