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Ann. Geophys., 22, 2555-2563, 2004
www.ann-geophys.net/22/2555/2004/
© European Geosciences Union 2004
Multipoint investigation of the source region of storm-time chorus
O. Santolík1,2, D. A. Gurnett1, and J. S. Pickett1
1Department of Physics and Astronomy, University of Iowa, Iowa City, IA 52242-1479, USA
2Now at Faculty of Mathematics and Physics, Charles University, Prague, and at Institute of Atmospheric Physics, Czech Academy of Sciences, Prague, Czech Republic
Abstract. In this case study we investigate the source
region of whistler-mode chorus located close to the
geomagnetic equator at a radial distance of 4.4 Earth radii.
We use measurements from the
four Cluster spacecraft at separations of less than a few
hundreds of km, recorded during the geomagnetic storm
of 18 April 2002.
The waveforms of the electric field fluctuations were
obtained by the WBD instruments in the frequency range
50Hz-9.5kHz. Using these data,
we calculate linear and rank correlation
coefficients of the frequency averaged
power-spectral density measured by the different spacecraft.
Those coefficients have been recently shown to decrease with
spacecraft separation distance perpendicular to the static magnetic field
cchor03 with a characteristic scale length of 100km.
We find this characteristic scale
varying between 60 and 200km for different
data intervals inside the source region.
We examine possible explanations for the observed large scatter of
the correlation coefficients,
and we suggest a simultaneously acting effect
of random positions of locations at which the individual chorus wave
packets are generated.
The statistical properties of the observations are
approximately reproduced by a simple 2-D model of the source
region, assuming a perpendicular half-width of 35km
(approximately one wavelength of the whistler-mode waves)
for the distribution of
power radiated from individual active areas.
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