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Ann. Geophys., 26, 1567-1579, 2008
www.ann-geophys.net/26/1567/2008/
© European Geosciences Union 2008
Sunward propagating Pc5 waves observed on the post-midnight magnetospheric flank
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. The overall focus on the driver of toroidal Pc5 waves has been on processes located at or
acting on the dayside magnetopause and dayside flanks of the magnetosphere. These
processes can generate waves that propagate tailward in the magnetosphere. However, an
increasing number of studies, both theoretical and experimental, have looked at waves
propagating sunward and that are caused by processes in the magnetotail. Here we present
an ultra low frequency (ULF) wave observed in the post-midnight/morning sector of the
magnetosphere at L=16 RE. The wave has a toroidal mode polarization. We estimate
the azimuthal wave number to m=3, consistent with a toroidal mode type pulsation. The
positive sign indicates that the wave is propagating sunward and this is confirmed by
looking at the Poynting flux of the wave. The frequency of the wave is not constant with
time but shows a small increase in the beginning of the event up to over 2.0 mHz. Then
the frequency decreases to 1.0 mHz. This decrease coincides with a drop in the total
magnetic field strength and we speculate if this is related to an observed reversal of
the sign of the interplanetary magnetic field (IMF) By-component. This event occurs
during relatively quiet magnetospheric conditions with a solar wind speed of
approximately 400 km/s. Thus this event is highly likely to be driven by a source in the
magnetotail and the change in frequency is an excellent example that the frequency of an
ULF wave may be modulated by changes of the plasma parameters on the resonant field line.
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