Kinetic treatment of the Jeans gravitational instability, with collisions taken into account, is presented. The initial-value problem for the distribution function which obeys the kinetic equation, with the collision integral conserving the number of particles, is solved. Dispersion relation is obtained and analyzed. New modes are found. Collisions are shown not to affect the Jeans instability criterion. Although the instability growth rate diminishes, the collisions they cannot quench the instability. However, the oscillation spectrum is modified significantly: even in the neighborhood of the threshold frequency =0 (separating stable and unstable modes) the spectrum of oscillations can strongly depend on the collision frequency. Propagating (rather than aperiodic) modes are also found. These modes, however, are strongly damped.

We examine the stability of the of gravitation has not been considered before. In Venus ionopause in light of the importance of addition we intend to take into acco,•nt the gravitation and curvature. Using a one fluid effect of the curvature of the VenusJan ionoapproximation for the equation of motion of the pause. The centrifugal acceleration of the ionoplasma, and ignoring the effects of neutrals, we spheric plasma, g = V2/R (where V 2 is the sum of obtain a dispersion relation that includes the the squares of plasma thermal speed and bulk flow effects of magnetic field, sheared plasma flow, speed, and R is the radius of curvature of the buoyancy, centrifugal force and magnetic tension interface) and the curvature of the external due to boundary curvature. We find that buoyancy magnetic field destabilize the Venusian ionoacts to neutralize the flute instability. As pause, whereas gravity has a stabilizing effect. Gravity is importamt in the Venus ionopause • is the frequency. The wavelengths • of the stability problem. The effects of gravity have disturbances are assumed to be small in comparibeen studied only by Chandrasekhar [1961] (his son with the radius R of the curvature of the results were used by Wolf• et al. [lqBO] for the interface, so that kR = 2•R/• >> 1. For such case of Venus) and only fo• some special cases. disturbances the interface may be considered as The general case of arbitrary orientation and plane; we take this p_lane as z --0, with the z magnitude of the magnetic field, plasma bulk axis antiparallel to g. For disturbances propavelocity, and the wave vector • in the presence gating along the (approximately) plane interface, k z --O. The undisturbed (equilibrium) values, Copyright 1984 by the American Geophysical Union. denoted by subscript zero,_are not assumed to be uniform, but the terms (B ß V)B,o , (V ß V)•o, Paper number 3A1700. V%o, etc. are eliminated øby using t%e corre-0148-0227/84/003A-1700502.00 sponding equilibrium equations. As a result, 997 998 Elphic and Ershkovich: Brief Report

Abstract. Selected problems of the solar wind -comet tail coupling that are currently accessible to quantitative analysis are reviewed. The model of a comet tail as a plasma cylinder separated by a tangential discontinuity surface from the solar wind is discussed in detail. This model is compatible with the well-known Alfv6n mechanism of formation of the comet tail. The stability problem of the comet tail boundary (considered as a discontinuity surface) is solved. Under typical conditions a comet tail boundary can undergo the Kelvin-Helmholtz instability. With finite amplitude the stabilizing effect of the magnetic field increases, and waves become stabilized. This model supplies a detailed quantitative description of helical waves observed in type-1 comet tails. A more general model of the tail boundary as a transition layer with a continuous change of the plasma parameters within it is also considered.This theory, in principle, enables us to solve one of the fundamental problems of cometary physics: the magnetic field of the comet tail can be derived from the observations of helical waves. This field turns out to be of the order of the interplanetary field. Various other considerations, discussed in this review, also support this conclusion.

Abstract. We compare the magnetic field topology in the Titan wake to an idealized picture of magnetic field lines draping about a conductive nonmagnetic obstacle. It is shown that in the inner part of the wake the magnetic field picture differs significantly from that expected for an idealized draping: The transverse magnetic field component rotates by 90 ø as compared with the direction of the upstream transverse magnetic field. Another difference is the existence of a deep magnetic field minima separating the inner and outer parts of the wake. Transverse magnetic field rotation can be explained neither by temporal changes in the upstream magnetic field nor by reconnection processes in the wake. We find that this behavior of the transverse magnetic field can be explained at least qualitatively if one assumes the existence of a small intrinsic magnetic field of Titan. An effective magnetic dipole of 1021 G cm 3 can account for the observed topology of the Titan magnetic wake. The origin of this field may be related to a residual magnetization of Titan's crust or to induction in a conducting ionosphere of the satellite. We present results of MHD simulations which support the above theoretical conclusion.

We have studied statistical distributions of the current density in the geomagnetic tail current sheet for different sets of local plasma parameters using Cluster data. It is shown that the electric current density calculated from 4‐point magnetic field measurements exhibits no correlation with the number flux of ions. We conclude that main current carriers in the magnetospheric system of reference are electrons.