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Ann. Geophys., 26, 1665-1670, 2008
www.ann-geophys.net/26/1665/2008/
© European Geosciences Union 2008
Frequencies of wave packets of whistler-mode chorus inside its source region: a case study
O. Santolik1,2, E. Macusova1,2, E. E. Titova3, B. V. Kozelov3,4, D. A. Gurnett1, J. S. Pickett1, V. Y. Trakhtengerts5,†, and A. G. Demekhov5
1University of Iowa, Iowa City, Iowa, USA
2permanently at Institute of Atmospheric Physics CAS, Prague and Charles University, Faculty of Mathematics and Physics, Prague, Czech Republic
3Polar Geophysical Institute, Apatity, Russia
4Department of Physics and Technology, University of Tromsø, Tromsø, Norway
5Institute of Applied Physics, Nizhny Novgorod, Russia
†deceased
Abstract. Whistler-mode chorus is a structured wave emission observed
in the Earth's magnetosphere in a
frequency range from a few hundreds of Hz to several kHz. We
investigate wave packets of chorus using high-resolution measurements
recorded by the WBD instrument on board the four Cluster spacecraft.
A night-side chorus event observed during geomagnetically disturbed conditions
is analyzed. We identify lower and upper frequencies for a large number of individual
chorus wave packets inside the chorus source region. We investigate how
these observations are related to
the central position of the chorus source which has been previously
estimated from the Poynting flux measurements.
We observe typical frequency bandwidths of chorus
of approximately 10% of the local electron cyclotron frequency.
Observed time scales are around
0.1 s for the individual wave packets.
Our results indicate a lower occurrence probability for lower frequencies
in the vicinity of the central
position of the source compared to measurements recorded closer to the outer boundaries of the source.
This is in agreement with recent research based on the backward wave oscillator theory.
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