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Ann. Geophys., 22, 2403-2411, 2004
www.ann-geophys.net/22/2403/2004/
© European Geosciences Union 2004
Cluster observations of high-frequency waves in the exterior cusp
Y. Khotyaintsev1, A. Vaivads1, Y. Ogawa2, B. Popielawska3, M. André1, S. Buchert1, P. Décréau4, B. Lavraud5, and H. Rème5
1Swedish Institute of Space Physics, Uppsala, Sweden
2Nagoya University, Japan
3Space Research Center, Warsaw, Poland
4LPCE/CNRS, Orleans, France
5CESR, Toulouse, France
Abstract. We study wave emissions, in the frequency range from above the
lower hybrid frequency up to the plasma frequency, observed
during one of the Cluster crossings of a high-beta exterior cusp
region on 4 March 2003. Waves are localized near narrow current
sheets with a thickness a few times the ion inertial length;
currents are strong, of the order of 0.1-0.5μA/m2
(0.1-0.5mA/m2
when mapped to ionosphere). The high frequency
part of the waves, frequencies above the electron-cyclotron
frequency, is analyzed in more detail. These high frequency waves
can be broad-band, can have spectral peaks at the plasma frequency
or spectral peaks at frequencies below the plasma frequency. The
strongest wave emissions usually have a spectral peak near the
plasma frequency. The wave emission intensity and spectral
character change on a very short time scale, of the order of 1s.
The wave emissions with strong spectral peaks near the plasma
frequency are usually seen on the edges of the narrow current
sheets. The most probable generation mechanism of high frequency
waves are electron beams via bump-on-tail or electron two-stream
instability. Buneman and ion-acoustic instability can be excluded
as a possible generation mechanism of waves. We suggest that high
frequency waves are generated by electron beams propagating along
the separatrices of the reconnection region.
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