ANGEO Annales Geophysicae ANGEO Ann. Geophys. 1432-0576 Copernicus Publications Göttingen, Germany 10.5194/angeo-19-1429-2001 Polarisation and propagation of lion roars in the dusk side magnetosheath Maksimovic M. 1 Harvey C. C. 2 Santolík O. 3 Lacombe C. 1 de Conchy Y. 1 Hubert D. 1 Pantellini F. 1 Cornilleau-Werhlin N. 4 Dandouras I. 2 Lucek E. A. 5 Balogh A. 5 DESPA, Observatoire de Paris, France CESR, Toulouse, France Department of Physics and Astronomy, University of Iowa, USA CETP/UVSQ, Vélizy, France Imperial College, London, UK 30 09 2001 19 10/12 1429 1438 Copyright: © 2001 M. Maksimovic et al. 2001 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://angeo.copernicus.org/articles/19/1429/2001/angeo-19-1429-2001.html The full text article is available as a PDF file from https://angeo.copernicus.org/articles/19/1429/2001/angeo-19-1429-2001.pdf

We present observations of &quot;lion roars&quot; 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 &quot;lion roars&quot;, 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 δ<i>B/B</i> 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.<br><br><b>Key words. </b>Magnetospheric physics (magnetosheath; plasma waves and instabilities) Radio science (radiowave propagation)