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Ann. Geophys., 25, 737-754, 2007
www.ann-geophys.net/25/737/2007/
© European Geosciences Union 2007
Comprehensive study of ULF upstream waves observed in the topside ionosphere by CHAMP and on the ground
B. Heilig1,2, H. Lühr1, and M. Rother1
1GeoForschungsZentrum Potsdam, Telegrafenberg, 1473 Potsdam, Germany
2Tihany Geophysical Observatory, Eötvös Loránd Geophysical Institute, Kossuth u. 91., 8237 Tihany, Hungary
Abstract. Based on magnetic field measurements from the satellite CHAMP, a detailed
picture could be obtained of the upstream wave (UW) distribution in the topside
ionosphere. The low, near-polar orbit of CHAMP, covering all local times,
allows the global distribution of this type of pulsation to be revealed. The
observations from space are compared to recordings of the ground-based MM100
meridional array covering the latitude range 66° to 42° in magnetic
coordinates. UWs show up very clearly in the compressional component of the
satellite magnetic field data, whereas on the ground, their signature is found
in the H component, but it is mixed with oscillations from field line resonant
pulsations. Here we first introduce a procedure for an automated detection of
UW signatures, both in ground and space data. Then a statistical analysis is
presented of UW pulsations recorded during a 132-day period, centred on the
autumn 2001 equinox. Observations in the top-side ionosphere reveal a clear
latitudinal distribution of the amplitudes. Largest signals are observed at the
equator. Minima show up at about 40° latitude. The coherence between ground
and satellite wave signatures is high over wide latitude and longitude
ranges. We make suggestions about the entry mechanism of UWs from the foreshock
region into the magnetosphere. The clear UW signature in satellite recordings
between −60° and 60° latitude allows for detailed investigations of the
dependence on solar wind conditions. We test the control of solar wind speed,
interplanetary magnetic field strength and cone angle on UWs. For the first
time, it is possible to derive details of the Doppler-shift effect by modifying
the UW frequency from direct observations. The results reconcile foreshock
wave generation predictions with near-Earth observations.
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