The nonlinear evolution of the electrostatic Kelvin-Helmholtz instability, resulting from velocitysheared plasma flows perpendicular to an ambient magnetic field, has been studied including Pedersen conductivity effects (i.e., ion-neutral collisions). We find that the Kelvin-Helmholtz instability develops in a distinctly different manner in the nonlinear regime with Pedersen coupling than without it. Specifically, we show that Pedersen coupling effects, in conjunction with a neutral wind and density gradient, (1) result in an increased time scale for Kelvin-Helmholtz instability wave growth, (2) inhibit Kelvin-Helmholtz vortex formation, (3) lead to nonlinear structures which can be described as "breaking waves," and (4) generate, in the nonlinear regime, small scale turbulence by means of secondary instabilities growing on the primary waves. We have also computed the spatial power spectra of the electrostatic potential and density fluctuations and find that there is a tendency for the potential and density spectra to become shallower when Pedersen conductivity effects are included. We compare our results with recent Dynamics Explorer satellite observations of velocity-sheared plasma flows in the high-latitude, near-Earth space plasma and find good agreement. Recently, much experimental [Basu et al., 1988; Weber and Buchau, 1981; Bythrow et al., 1984; Cerisier et al., 1985; Rodriquez and Szuszczewicz, 1984; Curtis et al., 1982; Baker et al., 1986; Vickrey et al., 1980] and theoretical (for recent reviews, see Keskinen and Ossakow [1983] and Kintner and $eyler [1985] and references therein) attention has been given to the origin of high-latitude ionospheric and magnetospheric plasma turbulence. The Kelvin-Helmholtz or velocity-shear driven instability can lead to both electric field and density fluctuations in the high-latitude near-Earth space plasma [see, for example, Kintner and Seyler, 1985]. Studies of velocitysheared flows in space plasmas can be divided into two groups depending upon whether plasma flow velocities are either parallel [Paper number 7A9077. 0148-0227/88/007A-9077505.00 Mishin, 1981; Lee et al., 1981; Walker, 1981; Keskinen and Huba, 1983] or perpendicular [Hallinan and Davis, 1970; Miura and Sato, 1978; Miura and Pritchett, 1982; Pritchett and Coroniti, 1984; Thompson, 1983] to the ambient magnetic field. Both cases have been studied in the MHD [Mikhailovskii, 1974; Sen, 1964; Southwood, 1968] and electrostatic [D'An•lelo, 1965; Smith and yon Goeler, 1968] limits. Furthermore, the velocity, in both cases, is usually taken to vary spatially transverse to the magnetic field in the electrostatic limit. In this study we restrict ourselves to sheared flows perpendicular to the geomagnetic field. Hallinan and Davis [1970] and Webster and Hallinan [1973] have attributed the small scale vortex configurations often seen near auroral arcs [Hallinan and Davis, 1970; Oquti, 1974] to be driven by a transverse Kelvin-Helmholtz or velocity shear driven instability. Kintner [1976] and Kelley and Carlson [197...

A magnetic‐field‐line‐integrated model of plasma interchange instabilities is developed for the high latitude ionosphere including magnetospheric coupling effects. We show that the primary magnetosphere‐ionosphere coupling effect is to incorporate the inertia of the magnetospheric plasma in the analysis. As a specific example, we present the first simulation of the instability in the inertial regime, i.e., νi « ω where νi is the ion‐neutral collision frequency and ω is the wave frequency. We find that the inertial E∼×B∼ instability develops in a fundamentally different manner than in the collisional case (νi » ω). Our results show that striations produced in the inertial regime are spread and retarded by ion inertial effects, and result in more isotropic irregularities than those seen in the collisional case.

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We have developed a dynamic numerical model of the plasma along an auroral field line in order to provide a vehicle for studying ionosphere‐magnetosphere coupling processes. The model is a multimoment, multifluid approximation of a gyrotropic plasma consisting of three species (electrons, hydrogen ions, oxygen ions) along a segment of auroral magnetic field line extending from an altitude of 800 km to 10 earth radii. We have performed simulations for the case of a current‐free polar wind equilibrium of the field line plasma and the case in which a large upward field‐aligned current is applied to the field line. In the former case, the agreement between our model and previous static results is reasonable given the differing boundary conditions inherent in the two cases. In the case of a field‐aligned current, we note that the flux tube plasma responds to the current on several time scales. After an initial rapid heating of the electrons due to precipitation in a converging magnetic field, electric field coupling of the electrons to the ions causes thermal oscillations of the flux tube plasma to persist on time scales of the order of 1 hour, illustrating the complicated nature of the response of a collisionless plasma when heat flow transport is treated in a dynamic manner.

The dynamic processes in the plasma along high‐latitude field lines plays an important role in ionosphere‐magnetosphere coupling process. We have created a time‐dependent, large‐scale simulation of these dynamics parallel to the geomagnetic field lines from the ionosphere well into the magnetosphere. The plasma consists of hot e− and H+ of magnetospheric origin and low‐energy e−, H+, and O+ of ionospheric origin. Including multiple electron species, a major improvement to the model, has allowed us for the first time to simulate the upward current region properly and to dynamically simulate the diodelike response of the field‐line plasma to the parallel currents coupling the ionosphere and magnetosphere. It is shown that return currents flow with small resistance, while upward currents produce kilovolt‐sized potential drops along the field, as concluded from satellite observations. The kilovolt potential drops are due to the effect of the converging magnetic field on the high‐energy magnetospheric electrons.