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Ann. Geophys., 21, 1111-1120, 2003
www.ann-geophys.net/21/1111/2003/
© European Geosciences Union 2003
Magnetospherically reflected chorus waves revealed by ray tracing with CLUSTER data
M. Parrot1, O. Santolík2, N. Cornilleau-Wehrlin3, M. Maksimovic4, and C. Harvey5
1LPCE/CNRS, 3A Avenue de la Recherche, ORLEANS, 45071 France
2Charles University, V. Holesovickach, Praha, 18000 Czech Republic
3CETP/UVSQ, 10/12 Avenue de L’Europe, Velizy, 78140 France
4LESIA, place Jules Janssen, Meudon, 92195 France
5CESR, 9 Av. Colonel Roche, Toulouse, 31029 France
Abstract. This paper is related to
the propagation characteristics of a chorus emission recorded simultaneously by
the 4 satellites of the CLUSTER mission on 29 October 2001 between 01:00 and
05:00 UT. During this day, the spacecraft (SC) 1, 2, and 4 are relatively close
to each other but SC3 has been delayed by half an hour. We use the data
recorded aboard CLUSTER by the STAFF spectrum analyser. This instrument
provides the cross spectral matrix of three magnetic and two electric field
components. Dedicated software processes this spectral matrix in order to
determine the wave normal directions relative to the Earth’s magnetic field.
This calculation is done for the 4 satellites at different times and different
frequencies and allows us to check the directions of these waves. Measurements
around the magnetic equator show that the parallel component of the Poynting
vector changes its sign when the satellites cross the equator region. It
indicates that the chorus waves propagate away from this region which is
considered as the source area of these emissions. This is valid for the most
intense waves observed on the magnetic and electric power spectrograms. But it
is also observed on SC1, SC2, and SC4 that lower intensity waves propagate
toward the equator simultaneously with the SC3 intense chorus waves propagating
away from the equator. Both waves are at the same frequency. Using the wave
normal directions of these waves, a ray tracing study shows that the waves
observed by SC1, SC2, and SC4 cross the equatorial plane at the same location
as the waves observed by SC3. SC3 which is 30 minutes late observes the waves
that originate first from the equator; meanwhile, SC1, SC2, and SC4 observe the
same waves that have suffered a Lower Hybrid Resonance (LHR) reflection at low
altitudes (based on the ray tracing analysis) and now return to the equator at
a different location with a lower intensity. Similar phenomenon is observed
when all SC are on the other side of the equator. The intensity ratio between
magnetic waves coming directly from the equator and waves returning to the
equator is between 0.005 and 0.01, which is in agreement with previously
published theoretical calculation of the growth rates with the particle
distribution seen by GEOS.
Key words. Magnetospheric physics
(plasma waves and instabilities) – Ionosphere (wave propagation) – Radio
science (magnetospheric physics)
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