JOURNAL OF GEopHYsIcAL RESEARCH, VOL. 90, NO. A1, PAGES 73-94, JANUARY 1, 1985 ELECTRON PLASMA WAVES UPSTREAM OF THE EARTH'S BOW SHOCK Catherine Lacombe, Andr• Mangeney, Christopher C. Harvey D•partement de Recherche Spatiale, Laboratoire Associ• 264 au CNRS Observatoire de Paris-Meudon, France Jack D. Scudder Laboratory for Extraterrestrial Physics Goddard Space Fligbt Center, Greenbelt, Maryland Abstract. Electrostatic waves are observed However, Gurnett et al. [1981] have noted that around the plasma frequency f in the electron this electrostatic noise is made up of an intense e foreshock, together with elec•ons backstreaming emission line, at f e' and of emissions above and from the bow shockUsing data from the sounder below f that the• call the "sidebands". They ' pe aboard ISEE 1, we show that this noise, previ- have incerpreted the whole as Langmuir waves with ously understood as narrow band Langmuir waves a bandwidth due to Doppler shift and non-linear more or less widened by Doppler shift or non- effects. linear effects, is in fact composed of two dis- Etcheto and Faucheux [1984] have shown that tinct parts: One is a narrow band noise, emitted the spectrum of the electron burst noise can be just above f e and observed at the upstream either narrow or wide, sometimes extending well boundary of t•e' electron foreshock. This compo- below or well above f , andthat the large nenthas been interpreted as Langmuir waves emit- bandwidth cannotbe e•%lained by the Doppler ted by a beam-plasma instability. We suggest that effect alone. They have shown that the spectral it is of sufficiently large amplitude and mono- width depends on the distance DIFF (defined on chromatic enough to trap resonant electrons. The Figure 1) between the spacecraft and the upstream other is a broad band noise, more impulsive than boundary of the electron foreshock, and that the the narrow band noise, observed well above and/or energy density of the waves is too small to give well below.fpe ., deeper in theelectron foreshock. rise to the non-linear effectspreviously in- The broad Dana noise has an average spectrum with voked, but too large to be explained as thermal a typical bi-exponential shape; its peak frequen- noise. cy is not exactly equal to f and depends on the The polarisation studies of Filbert and pe Debye length. This peak frequency also depends on Kellogg [1979], Anderson et al. [1981] and the velocity for which the electron distribution Etcheto and Faucheux [1984] show that the elec- has maximum skew. An experimental determination tric field of this noise is mainly directed along of the dispersion relation of the broad band the interplanetary magnetic field. noise shows that this noise, as well as the nar- In this paper we present a statistical study row band noise, may be due to the instability of of the main characteristics of the electron burst a hot beam in a plasma. noise observed by the sounder on ISEE 1, and we present observations of the associated solar wind 1. Introduction and backstreaming electrons. In particular, we show that the electron burst noise consists of Electrostatic waves around the electron plasma two components: frequency have been observed upstream of the The first is a narrow band noise, emitted just Earth's bow shock for more than 12 years [Scarf above the electron plasma frequency f at the et al., 1971; Fredricks et al., 1971; Filbert and upstream boundary of the electron P•reshock, Kellogg, 1979; Anderson et al., 1981; Parks et which has been interpreted as Langmuir waves al., 1981; Etcheto and Faucheux, 1984]. Similar excited by a two-stream instability of fast elec- electrostatic waves have been observed by Voyager trons. We show that this narrow band noise is upstream of the Jupiter's bow shock [Gurnett et intense enough to trap the electrons backstream- al., 1981] and upstream of Saturn [Scarf et al., ing from the bow shock. 1981; Scarf et al., 1982]. These waves are corre- The second is a broadband impulsive noise, lated with the presence of electrons backstream- observed deeper in the electron foreshock, above ing from the bow shock: after the work by Filbert and/or below f , at frequencies which cannot be pe and Kellogg [1979], they are observed when the explained by Eangmuir waves suffering a Doppler spacecraft is located on a line of force of the effect in the solar wind. The broadband noise has interplanetary magnetic field connected to the a characteristic bi-exponential average spectrum, bow shock surface, as shown on Figure 1. with a peak frequency and run-offs which depend These electrostatic waves, called electron on the Debye length. This peak frequency f is burst noise by Harvey et al. [1979], are gene- notexactly equal to f-e' and If-f-^l may b• as rally considered as emitted by a beam-plasma large as5 kHz. The shaPpe c •= of the spectrum depends instability at the electron plasma frequency f•e. on the connexion depth, and the wave number tends [see, for example, Fitzenreiter et al., 198 to increase with the observed frequency. The outstanding questions to be answered are: Copyright 1985 by the American Geophysical Union. 1. How is the broadband noise generated? 2. Does this noise depend only on the local Paper number 4A8128. distribution function of the backstreaming elec- 0148-0227/85/004A-8128505.00 trons, or does it depend also on the level of the 73