Pergamon www.elsevier.com/locate/asr Available online at www.sciencedirect.com mClRNCR DINRCT. doi: 10.1016/SO273-1177(03)00271-O FINE STRUCTURE OF THE POLAR CUSP AS DEDUCED FROM THE PLASMA WAVE AND PLAi3M.A MEASUREMENTS J. Blecki’, S. Savin2, N. Comilleau-Wehrlin3, K. Kossacki’, M. Parrot4, H. Rothkaehl’, K. Stasiewicz5, R.Wronowski’, 0. Santolik’, J-A. Sauvaud7. ‘Space Research Centre PAS, (IO- 716 Warsaw, Bartycka 18A, Poland 2Space Research Institute RAS,1 I 7810, Profsoyusnaya 84/32, Moscow, Russia 3 CETP/Ch!RS, O/l 2 Avenue de LIEurope, F- 78140 Velizy, France 4LPCE/CNRS, 3A, Ave. de la Recherche Scientifique, F-45071 Orleans Cedex 2, France ‘Space Physics Institute, Box 537, SE-751 21 Uppsala, Sweden 6Charks University, V Holesovichach 2, 18000 Prague 8, Czech Republic. 7CESR/CNRS, 9 Avenue du Colonel Roche, B.P. 4346, F-31 028 Toulouse Cedex 4,‘France ABSTRACT Plasma waves are one of the most significant features of plasma. They are a very sensitive indicator of the boundaries and inner structure of different regions in space plasmas. The data gathered onboard Interball 1 and Magion 4 in the polar cusp indicate that plasma wave intensity changes very rapidly with a characteristic period of the order of l-50 seconds, which differs from the satellite rotation period. Comparison with plasma data suggests that it could be associated with real inner structure of the polar cusp. A similar effect has been seen in the data from Prognoz 8 and Freja satellites. CLUSTER data also confirm these effects. The analysis of the wave and plasma data from these satellites associated with tine (filamentary) structures in the cusp is given in this paper. This kind of filamentary structure plasma is often associated with nonlinear stage of the current instabilities or nonlinear evolution of the Alfven waves. The polar cusp can be used as a laboratory for studies of the plasma non-linear processes. Discussion of possible source of this fine structure is presented at the end of this paper. 0 2003 COSPAR. Published by Elsevier Ltd. All rights reserved. INTRODUCTION Plasma turbulence is one of the most significant features of the polar cusp. Alfven, lower hybrid, electron and ion cyclotron waves are always observed there. Higher frequency waves at Langmuir and upper hybrid frequencies are also often seen in the cusps (Potellette et al., 1990; Blecki et al.,1998; Savin et al., 1999). Another general feature of a magnetized plasma is a tendency to create filamentary structures in the field aligned currents (Perrat, 1991). This tilamentation can be seen by optical and X-ray observations of the solar chromosphere and corona, in the cometary’s tails and on the Earth in the aurora. Magnetospheric plasma indicates also a tendency to form subtle structures in the current flowing. along magnetic field line. The polar cusp is a location where field aligned currents are present ‘(Yamauchi et al., 2000). The information given by waves can be useful to determine the physical conditions in these plasma structures. Waves give information on the microphysics of the processes in plasmas. The temporal and spatial resolutions obtained from the wave spectra are the highest. This fact allows us to use the wave spectra as a main tool to study fine structure of the polar cusp. Prognoz-8, Magion-4, Interball Tail Probe and CLUSTER with their orbits crossing the polar cusps - northern at high and southern at much lower altitudes, additionally Freja at lower orbit, gives complementary information about structure of polar cusps along their altitudes. Examples of observation of these types ‘of Adv. Space Res. Vol. 32, No. 3, pp. 315321.2003 8 2003 COSPAR. Published by Elsevier Ltd. All rights reserved Printed in Great Britain 0273-l 177/$30.00 + 0.00