A hybrid simulation model with kinetic ions, massless fluid electrons, and phenomenological resistivity is used to study the perpendicular configuration of the bow shocks of the earth and other planets. We investigate a wide range of parameters, including the upstream Mach number, electron and ion beta (ratios of thermal to magnetic pressure), and resistivity. Electron beta and resistivity are found to have little effect on the overall shock structure. Quasi-stationary structures are obtained at moderately high ion beta (/3i '• 1), whereas the shock becomes more dynamic in the low ion beta, large Mach number regime (/3i '• 0.1, MA > 8). The simulation results are shown to be in good agreement with a number of observational features of quasi-perpendicular bow shocks, including the morphology •'• of the reflected ion stream, the magnetic field profile throughout the shock, and the Mach number dependence of the magnetic field overshoot.

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Simulations of a high Mach number shock with parameters typical of the earth's bow shock have been performed using a hybrid (particle ions, fluid electrons) code. The simulations reproduce the observed ion reflection and overshoots in the magnetic field and density. These features are shown to be closely associated with ion gyration.

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Recent satellite observations of electrostatic and magnetic noise in the distant magnetotail (Gurnett et al., 1976) can be explained by the excitation of the lower-hybrid-drift instability. In particular, it is shown that (1) existence conditions for the lower-hybrid-drift instability can be met, (2) the observed frequency spectra and polarization are in good agreement with the predictions of linear theory, and (3) the observed amplitudes of fluctuations are consistent with the nonlinear theory of this mode. Moreover, the observation of this instability suggests that the anomalous transport properties associated with these waves, which are important in many laboratory devices, may play a crucial role in the macroscopic evolution of magnetotail processes such as field line merging, tearing instabilities, or 'fireballs.' INTRODUCTIONAn important aspect of magnetospheric physics is understanding the abundant processes associated with microscopic plasma turbulence. Some of these turbulent microscopic processes result in anomalous plasma transport properties (i.e., processes not explainable in terms of classical Coulomb collisions) and hence greatly influence the macroscopic behavior of the magnetospheric plasma. Many of the most dramatic phenomena in the magnetosphere (e.g., magnetic field line reconnection, magnetic substorms, and aurorae) involve turbulent microscopic plasma phenomena in an integral fashion.We recently proposed [Huba et at., 1977] that the lowerhybrid-drift instability [Krall and Liewer, 1971], an instability of considerable importance in several magnetic fusion confinement systems [e.g., Davidson et at., 1976; Comrnisso and Griem, 1976], would be operative over large regions of the magnetotail, and, furthermore, may play a significant role in the development of field line reconnection as a source of anomalous resistivity. In this paper we demonstrate that recent experimental measurements of magnetotail turbulence support our theoretical prediction that the lower-hybrid-drift instability is active in the magnetotail. However, the one-dimensional laminar equilibrium used by Huba et al. [1977] is overly simplistic in modeling the plasma sheet, since experimental observations indicate that it is generally a turbulent medium [Frank et at., 1976; Coroniti et al., 1977]. Rather, localized spatial gradients within a much broader plasma sheet are considered to excite lower-hybrid-drift turbulence in the magnetotail. Gurnett et at. [1976] have made detailed satellite observations of plasma turbulence in the distant magnetotail (23-46 RE) and have found broadband electrostatic noise, magnetic noise, and electrostatic electron cyclotron noise. The broadband electrostatic turbulence is the most intense and frequently occurring type and is strongest in the frequency range fit << O• • fie. The turbulence is observed in regions of strong plasma and magnetic field gradients in the plasma sheet, where reconnection and/or tearing modes are believed to exist. The magnetic noise is observed in the same spatial regions...