Annales Geophysicae www.ann-geophys.net 0992-7689 1432-0576 21 7 2003 10.5194/angeo-21-1457-2003 http://www.ann-geophys.net/21/1457/2003/ http://www.ann-geophys.net/21/1457/2003/angeo-21-1457-2003.html http://www.ann-geophys.net/21/1457/2003/angeo-21-1457-2003.pdf 1457 1465 0000-00-00 On the existence of Alfvén waves in the terrestrial foreshock J. P. Eastwood A. Balogh E. A. Lucek C. Mazelle I. Dandouras Space & Atmospheric Physics Group, Blackett Laboratory, Imperial College, Prince Consort Rd, London SW7 2BW, UK CESR/ CNRS, 9, Avenue du Colonel Roche F-31028 Toulouse Cedex 4, France The terrestrial foreshock is characterised by the existence of large amplitude ultra low frequency waves. The majority of such waves are observed to be left-handed in the spacecraft frame, but are in fact intrinsically right-handed and have been identified as fast-magnetosonic waves. More rarely observed are waves that are right-handed in the spacecraft frame. Cluster four spacecraft observations of such waves are presented and analysed using multi-spacecraft techniques; in particular the k-filtering/wave telescope technique is used. The waves are found to be left-handed and propagating sunwards in the plasma rest frame, and are, therefore, identified as Alfvénic. The convection of the waves anti-sunward in the solar wind flow causes the observed polarisation to be reversed. Generation mechanisms are discussed.<br><br><b>Key words. </b>Interplanetary physics (MHD waves and turbulence; planetary bow shocks) – Space plasma physics (wave particle interactions)