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Ann. Geophys., 23, 919-929, 2005
www.ann-geophys.net/23/919/2005/
© European Geosciences Union 2005
Evidence of transverse magnetospheric field line oscillations as observed from Cluster and ground magnetometers
A. K. Sinha1, T. K. Yeoman1,*, J. A. Wild1, D. M. Wright1, S. W. H. Cowley1, and A. Balogh2
1Department of Physics and Astronomy, University of Leicester, LE1 7RH. UK
2Imperial College, London, UK
*Permanent address: Indian Institute of Geomagnetism, Mumbai, India
Abstract. The dynamic spectrum of ULF waves from magnetic field data obtained by the
elliptically orbiting Cluster satellites (with an apogee of 119000km,
perigee of 19000km and the orbital period of 57h) have been prepared in
the frequency range 0 to 120mHz when the satellite was near its perigee.
The existence of field line oscillations, with increasing frequency in the
inbound sector and decreasing frequency in the outbound sector, is seen in
the transverse components, indicating the presence of independently
oscillating local magnetic flux tubes in the form of transverse standing
Alfvén waves. The results show that toroidal and poloidal modes are
excited simultaneously. The analysis of simultaneous ground magnetometer
data at the footprint of the satellite suggests that these modes are also
excited due to coupling to magnetospheric waveguide modes. The clear
signature of a resonant fundamental mode is seen in the ground data whereas
Cluster detects a harmonic of this frequency. Lower frequency modes
indicative of waveguide oscillations are seen in both the ground data and
the compressional field at Cluster.
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