A two ion species system of fluid equations is used to investigate mechanisms responsible for transverse accelerations of plasma releases into high Mach number flows, such as that of the December 1984 Active Magnetospheric Particle Tracer Experiment (AMPTE) release of barium ions into the solar wind. We extend previous work in single ion species systems by developing a set of modified MHD equations which incorporates both the unmagnetized nature of the released ions and the interactions of the two ion fluids. An analysis of this system of equations suggests two mechanisms that could cause transverse shifts of the plasma release. One of these is based on a magnetic drift wave, the other on a hybrid cyclotron mode. Numerical simulations produce clearly observable shifts in the case of each mechanism. The modified two‐fluid MHD equations, simulation results, and physical mechanisms are discussed, and their relevance to the AMPTE barium release is examined.

The plasma that enters the region of the magnetotail current sheet can consist of multiple populations, each having a different spatial and temporal history. Processes remote from the current sheet can affect the distribution function of each component. In turn, these populations determine the structure of the current sheet. This interaction provides a purely kinetic mechanism by which “remote” processes can influence the current sheet structure. We investigate the effects of such multicomponent distributions on the current sheet, using self‐consistent, one‐dimensional test particle simulations. We model the system by separating the incoming plasma distribution into two components: a slowly drifting Maxwellian primary population and a low‐density secondary population. We then examine the influence of the secondary component by varying the energy and maximum pitch angle of this population. We find that changes in a small fraction of the incoming particle distribution can strongly modify the current sheet structure. In particular, increasing the field alignment or energy of 10% of the incoming particles can lead to significant thinning of the current sheet. This “kinetic thinning” is distinct from forces related to the large‐scale electric and magnetic fields and suggests that remote processes can have a strong influence on the structure of the magnetotail.

The unanticipated acceleration of the December, 1984 AMPTE release transverse to the solar wind velocity is examined by a modified MHD fluid simulation. By taking into account the unmagnetized nature of the released ions, some of the essential features of the experiment are reproduced. These include a shift transverse to the solar wind as well as the subsequent downwind acceleration. The modified MHD equations, simulation results, and physical mechanisms are discussed.

Particle acceleration near an X line in the magnetotail and the resulting ion distribution functions in the central plasma sheet are studied by using 2«-dimensional test particle simulations. The electric and magnetic fields are taken from a threedimensional global magnetohydrodynamic (MHD) simulation model describing the solar wind-magnetosphere interaction. A southward interplanetary magnetic field is chosen as the input solar wind condition. Test particles are generated from a model ion source distribution consistent with the macroscopic MHD parameters in the lobe region. The particles are calculated earthward and tailward of the X line. It is found that ion distributions consist of two main components: a low-energy population similar to the input distribution and a high-energy population. The former corresponds to particles that do not cross the region in which the Bx field reverses direction, while the latter consists of particles which cross the midplane, gaining energies on the order of keV. The spatial dependence of the relative population of low-energy versus high-energy components is discussed. The details of the acceleration process are determined from the motion of test particles in the simulation. 1. Palmadesso, 1986], and ion distribution functions have been modeled by using test particles [Lyons andSpeiser, 1982; Chen etal., 1990a, b; Burkhart and Chen, 1991; Ashour-Abdalla etal., 1991, 1993]. Particle energization and distribution functions in X line magnetic field configurations have also received extensive attention [Wagner etal., 1981; Curran etal., 1987; Curran and Goertz, 1989; Martin and Speiser, 1988; Doxas etal., 1990; Burkhart etal., 1990; region in which the current and magnetic field were iterated until they were self-consistent, satisfying J -(c/47r)V x B. In the above works, uniform cross-tail electric fields (Ey) are imposed, which can be transformed away in simple onedimensional cases. In a different approach, Sato etal. [1982] investigated the acceleration of test particles using electric and magnetic fields obtained from a magnetohyckodynamic (MHD) simulation of an X-type reconnection region. This work used an isolated X line structure, and reconnection was forced by injecting magnetized plasma toward the neutral line. Scholer and Jamitzky [1989] and Birn and Hesse [1994] calculated test particle trajectories and their energetics in time-dependent fields during plasmoid formation in two and three dimensions, respectively. In the work of Scholer and Jamitzl• [1989], reconnecfion is triggered in an initial current sheet by a localized resistivity. In the work of Birn and Hesse [1994], a plasmoid is formed in a model magnetotail in response to introducing anomalous resistivity. Test particle trajectories are then calculated in the simulated electric and magnetic fields. In order to incorporate largeo,.•,• •,•ao encompassing different regions, Ashour-Abdalla et al. [1993] used a two-dimensionalreduction of the Tsyganenko [1989] model in steady state with an imposed uniform cross-tail ...

Hybrid simulations (kinetic ions, massless fluid electrons) have been performed of the magnetotail current sheet for parameters characteristic of the quiet magnetotail (υD, bn < 1). Here bn = Bn/B0,υD = (cEy/Bn)/υi,Bn is the north / south (z) magnetic field, B0 the asymptotic value of the tailward (x) field and υi the ion thermal speed, where the magnetospheric coordinate system is used. It is shown that the current sheet is more stable than in previously examined cases with high υD and / or large bn. A clear single reversal in Bx persists over the entire simulation and a By component is created, with a slow rate of growth (hundreds of Ωi−1, where Ωi = eB0/miC). This is many minutes in real time. It is suggested that the waves are due to an electromagnetic instability of the cross‐field ion current.