NTC Radiation reflected on Magnetopause Surface: Observations from Cluster Large 
Scale Configuration

S. Grimald, P. M. E. Decreau, S. Boardsen, P. Canu, F. El-lembdani Mazouz, 
J. Green, K. Hashimoto, A. Rochel, X. Suraud, X. Vallieres


Non Thermal Continuum radiation (NTC) is an electromagnetic wave of weak
intensity, steady in time. Observations of terrestrial NTC radiation, below ~100
kHz, can be classified in two classes. A first class corresponds to wave
frequencies above the magnetosheath plasma frequency, the radiation escaping in
the interplanetary medium. Corresponding spectral characteristics are a series
of narrow banded emissions. A second class corresponds to wave frequencies below
the plasma frequency of the magnetosheath. The radiation bounces at the
magnetopause and is trapped in the magnetospheric cavity. This class of NTC is
often observed in frequency-time spectrograms as a wide and continuous frequency
band. In this paper, we present an event of bounced radiation whose spectral
characteristics are similar to those of escaping radiation.

The Cluster mission, which has been launched during the 2000 summer, is a
constellation of four identical satellites. The tetrahedral disposition of the
satellites allows a spatial-temporal study of the structures they crosses. The
two main purposes of the WHISPER experiment are to record the natural waves in
the bandwidth 2-83 kHz and to make a diagnostic of the electron density using
the sounding technique. The various working modes and the Fourier transforms
calculated on board provide a good time and frequency resolution and allow us
to: (i) detect the fine structure of NTC emissions, (ii) derive directivity
properties, based on spin modulation of the received electric field.

On July, 16, 2005, the four Cluster satellites observe NTC radiation in the
plasmapause vicinity. A series of distinct and parallel NTC narrow bands are
observed in frequency-time spectrograms above the local plasma frequency.
Despite the very large separation between the satellites (10 000 km), these
bands and their variations in frequency appear at the same time on each
satellite spectrogram. The phenomenon is also observed in data of IMAGE (nearby
Cluster) and Geotail (placed downstream in the tail). By using (1) spin
modulation properties, (2) ray tracing, we show that observations can be
explained by the following scenario: radiation emitted by a source region placed
near equator in the dawn side of the outer plasmasphere is bounced at the
magnetopause, and can be observed, showing similar patterns, by the different
platforms.

_______________
European Geosciences Union General Assembly, Vienna, Austria, 2-7 April 2006