Kinetic modeling of charged particle cloud expansion and emission in magnetic and electric fields

Abstract

The kinetic theory of unsteady charged-partitle fluxes in electric and magnetic fields is dealt with. An attempt is made to extend the results of Narasimha ( J. Fluid Meth. 12, 294–305, 1962) and Burgan et al. ( J. Plasma Phys. 29, 139–142. 1983). These authors studied the neutral and electron gas expansion into a vacuum, respectively. Burgan et al. ( J. Plasma Phys. 29, 139–142. 1953) considered the Vlasov equation, coupled self-consistently with the Poisson equation. The magnetic field effects were omitted. Gunko and Ponomarjov ( Vestnik St-Petersburg State Univ. Ser. 1, No. 2. 89–94, 1993; Astron. Nachr. 316(1), 17–21, 1995) carried out an analysis of the magnetic field effect for a special case of particle emission. A general method is proposed for simulating the charged-particle cloud expansion and the emission from sources in both electric and magnetic fields based on the kinetic approach. It consists of the representation of any cloud or emission source by the superposition of point-like clouds or sources with the corresponding initial particle distribution. As examples, the expansion of the following spatial domains filled by charged particles are analyzed : the semi-space, the plane layer of finite thickness, the circular cylinder, and the spherically symmetric cloud. In all the cases considered, Maxwell initial velocity distribution has been used for homogeneous magnetic and electric fields. It is found that in the case of the spherical cloud expansion, the cloud configuration at large times tends to an ellipsoid whose major axis expands along the magnetic field. Obtained results support the suggestion that this method has addressed the problem of plasma-cloud expansion into a uniform magnetic. field, with emphasis on the dynamics of lower density parts which appear to be highly structured in the form of field aligned striations (Davis et al., Planet. Space Sci. 22, 67–78, 1974). Different specific situations for a small charged-particle source is considered, by assuming that the effect of induced fields is very small compared to that of the ambient magnetic field in rare-lied plasma. It should be noted that the last condition is satisfied, for instance, in the satellite altitude regime (above 200 km), where the source velocity is well above the thermal velocity of ions and very small compared to the thermal velocity of plasma electrons. So that, any polarization electric field is effectively neutralized by faster electrons. The analytical results describe a flute formation along the edges of the charged-particle cloud that have been created by a small source moving parallel or approximately parallel to the ambient magnetic field. For the source in motion perpendicular or approximately perpendicular to the ambient magnetic field, developing field aligned striations of surfaces of constant charged-particle concentration are obtained.