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Ann. Geophys., 19, 1429-1438, 2001
www.ann-geophys.net/19/1429/2001/
© European Geosciences Union 2001
Polarisation and propagation of lion roars in the dusk side magnetosheath
M. Maksimovic1, C. C. Harvey2, O. Santolík3, C. Lacombe1, Y. de Conchy1, D. Hubert1, F. Pantellini1, N. Cornilleau-Werhlin4, I. Dandouras2, E. A. Lucek5, and A. Balogh5
1DESPA, Observatoire de Paris, France
2CESR, Toulouse, France
3Department of Physics and Astronomy, University of Iowa, USA
4CETP/UVSQ, Vélizy, France
5Imperial College, London, UK
Abstract. We present observations
of "lion roars" obtained in the magnetosheath by the Spectrum
Analyser (SA) of the Spatio-Temporal Analysis of Field Fluctuations (STAFF)
experiment aboard Cluster. STAFF-SA calculates, in near real time, the complete
auto- and cross-spectral matrix derived from three magnetic and two electric
components of the electromagnetic field at 27 frequencies in the range of 8 Hz
to 4 kHz. This allows the study of the properties of whistler mode waves and
more particularly, the properties of "lion roars", which are intense,
short-duration, narrow-banded packets of whistler waves. Their presence is
favoured by the magnetic field troughs associated with mirror mode structures.
During two short periods of well-defined mirror modes, we study the depth
δB/B of the magnetic troughs, and the direction of
propagation of the lion roars. During the first period, close to the
magnetopause, deep magnetic troughs pass the satellites. Lion roars are then
observed to propagate simultaneously in two directions, roughly parallel and
anti-parallel to the magnetic field: this probably indicates that during this
period, the satellites were within the successive source regions of lion roars.
For the second period, far from the magnetopause, the magnetic troughs are less
deep. Lion roars are propagating in only one direction, roughly anti-parallel
to the magnetic field, suggesting that the source regions are more distant and
predominantly on one side of the satellites.
Key words. Magnetospheric physics (magnetosheath;
plasma waves and instabilities) Radio science (radiowave propagation)
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