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The broadbaaad electrostatic waves inside the electron foreshock region well away from its boundaries were explained as a beam mode for which the beam velocity is lower than the thermal velocity of the background plasma. We discuss here the nonlinear saturation of this instability. The waves propagating along the beam are stabilized by a plateau formation on the electron distribution function.The waves that propagate at an angle to the beam are stabilized in a different way. In the course of the development of the instability the beam velocity spreads and an initially negative energy mode, which is initially unstable, experiences increased damping by the beam particles and finally is stabilized due to this effect. We give an estimation of the wave energy in both cases, and we present a numerical modelling to substantiate our analytical estimates. The saturated wave energy level is found to be comparable in both cases and agrees with the levels observed in the foreshock. 23,473 23,474 SOTNIKOV ET AL.: NONLINEAR PLASMA WAVES IN THE FORESHOCK Schriver, D. and M. Ashour-Abdalla, Broadband electrostatic noise due to field-aligned currents, J. Geoph•]s. Res.,

We propose a scenario for radiation of electromagnetic whistlers as a result of the injection of a modulated electron beam into the ionosphere. We show that for plasma parameters typical of the ionosphere and electron beam parameters typical of electron guns carried into space aboard rockets or spacecraft, the interaction can result in the instability of electrostatic waves driven by Cherenkov resonance between the waves and the beam. A linear stability analysis was carried out for two limiting cases: a wide and a narrow electron beam; the wave excitation is shown to develop at typical distances of the order of 1 km. Nonlinear evolution of the instability was investigated by a combination of analytical and numerical methods. Electron beam energy is efficiently transformed into the electrostatic mode. The mechanism of electromagnetic mode generation is then dipole radiation from the electron beam, which is strongly bunched as a result of the electrostatic instability. The efficiency of radiation is a strong function of the ratio of the electron beam velocity to electron Alfven velocity (Vae = C O)ce/O)pe); the possibility of increasing the efficiency of radiation is discussed.