Pergamon www.elsevier.com/locate/asr Adv. Space Rex Vol. 28, No. 11, pp. 1643-1648,2001 0 2001 COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0273-I 177101 $20.00 + 0.00 PII: SO273-1177(01)00479-3 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONM BROADBAND ELECTROSTATIC NOISE DUE TO NONLINEAR ELECTRON-ACOUSTIC WAVES S.V. Singh, R.V. Reddy, and G.S. Lakhina Indian zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Institute of Geomagnetism, Dr. Nanabhai M oos Marg, Colaba Mumbai-400005, India ABSTRACT Nonlinear propagation of electron-acoustic waves is examined in an unmagnetized, four-component plasma consisting of hot Maxwellian electrons, fluid cold and beam electrons and ions. Solitary struc- tures which are a possible final stage of the electron-acoustic wave growth are obtained. The soliton amplitude and width are numerically obtained. The results are compared with the spiky structures of the broadband electrostatic noise observed in the aurora1 region of the Earth’s magnetosphere. The model predicts parallel electric fields - (10-400) mV/m with typical half widths of the structures - a few Debye lengths. 0 200 1 COSPAR. Published by Elsevier Science Ltd. All rights reserved. INTRODUCTION Satellite measurements in the aurora1 and other regions of the magnetosphere have shown btirsts of broadband electrostatic noise (BEN) emissions with frequencies upto and higher than the electron plasma and cyclotron frequencies. High time resolution data analysis have indicated that BEN consists of small scale, large amplitude, magnetic field aligned electric fields in the aurora1 acceleration region (Temerin et al., 1982, Bostrom et al., 1988, Mozer et al., 1997, Ergun et al., 1998, Bounds et al., 1999, Pottelette et al., 1999), in the plasma sheet boundary layer (PSBL) (Matsumoto, 1994), in the Earth’s high altitude polar magnetosphere (Franz et al., 1998), in polar cap boundary layer ,l’CBL) (Tsurutani et al., 1998), and on cusp field lines (Catell et al., 1999). These large amplitude spiky structures in the parallel electric field have been interpreted in terms of either solitons (Temerin et al., 1982) or phase space holes (Omura et a1.,1994). The observations have shown two different types of solitary potential structures, i.e., ion and electron solitary potential structures propagating parallel to the magnetic field. These ion and electron solitary potential structures propagate at N lo-300 km/s and - 500-5000 km/s, respectively. The parallel scale sizes of these structures are in the range of 100-1000 m and the electric field amplitude can be of as high as a few hundreds mV/m. A detailed study of the properties of the broadband electrostatic noise in the dayside aurora1 zone has been presented by Dubouloz et al., (1991a). The presence of high frequency broadband electrostatic noise and large amplitude of electric field suggest that nonlinear effects play an important role in the generation of BEN. Dubouloz et al., (1991b, 1993) studied the electron-acoustic solitons (EAS) with two electron-component (cold and hot) and motionless ions and showed that the high frequency broadband electrostatic noise in dayside aurora1 zone can be generated by the EAS. The effect of upward propagating electron beams which are commonly observed in the dayside aurora1 zone was not taken into consideration. Berthomier et al., (1998) studied the characteristics of ion acoustic 1643