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Ann. Geophys., 22, 303-315, 2004
www.ann-geophys.net/22/303/2004/
© European Geosciences Union 2004
Compressional waves in the Earth's neutral sheet
M. Volwerk1,3, W. Baumjohann1, K. H. Glassmeier2, R. Nakamura1, T. L. Zhang1, A. Runov1, Z. Vörös1, B. Klecker3, R. A. Treumann3, Y. Bogdanova3, H.-U. Eichelberger1, A. Balogh4, and H. Rème5
1IWF, ÖAW, Graz, Austria
2TU Braunschweig, Germany
3MPE Garching, Germany
4Imperial College, London, UK
5CESR, Toulouse, France
Abstract. Compressional waves in the Earth's current sheet, driven by the high-speed
plasma flows connected to substorms, are investigated using the Cluster
magnetometer and plasma instrument. During the time that Cluster had its
apogee in the magnetotail (July through October 2001), we have studied 5
events in detail. We find compressional waves in the 30–60mHz band, at a
spectral power density that is dependent on when and where the event is
observed. There is a difference of two orders of magnitude in power density
between waves at substorm onset and waves during quiet times. Strong plasma
flows are the driver of the wave power. The spacecraft location in the
current sheet is also important for the spectral power density. Having four
spacecraft available we can discern spatial from temporal variations. We have
determined the propagation direction of the waves in the 30–60mHz band and
found that in the Cluster rest frame they propagate in the same direction as
the plasma flow at an angle 30° < ∆φ < 40° with respect to the plasma flow direction in the spacecraft' rest frame.
Key words. Magnetospheric physics (magnetotail; MHD
waves and instabilities; plasma sheet)
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