Elden C. Whipple

A two-dimensional, time-dependent, near-earth magnetotail

Abstract The plasma particle distributions in the near-earth region of the earths magnetosphere (6–12 R e ) appear to have a complicated structure both in velocity space and real space. Recent observations and theoretical work have emphasized the role of non-isotropic particle distributions in this region. A kinetic approach may be necessary to describe field/plasma configurations in the near-earth magnetotail. We present such an approach for obtaining slowly time-dependent, self-consistent plasma and magnetic field configurations for this region, using anisotropic particle distributions. At present, a two-dimensional configuration is used. The time variation of the plasma/field configuration is assumed to be driven by a slowly varying, externally imposed electric field (representing, for example, a change in solar wind conditions). The numerical approach involves solving a two-dimensional non-linear Poisson equation for the magnetic vector potential at each time step. The local current density is calculated from particle velocity distributions under the assumption of adiabatic particle motion. The obtained equations of state will be compared with those of double-adiabatic theory. Numerical examples of self-consistent responses of the near-earth plasma and field to an externally imposed electric field pulse will be presented.

A self-consistent model for the particles and fields upstream of an outgassing comet: 1. Gyrotropic and isotropic ion distributions

The magnetic field and solar wind flow parameters in the vicinity of a weakly outgassing comet are determined using a self-consistent model which treats the cometary ions kinetically. Two different assumptions are made concerning the cometary ion distribution function; the pickup cometary ions form either a velocity space gyrotropic ring distribution or a velocity space Isotropic shell distribution in the solar wind frame of reference. The individual currents due to the solar wind and cometary ions, which determine the magnetic field, are calculated in each case. Using theoretically determined parameters which reflect the conditions expected at comet Kopff (a typical short-period comet and one possible target for the future CRAF/Cassini mission) for various heliocentric distances, we are able to determine how the global field and flow characteristics are influenced by the nature of the cometary ion distribution function.

Generalized adiabatic theory applied to the magnetotail current sheet

We use the generalized first adiabatic invariant, an extension of the magnetic moment for regions of large field gradients, to treat particles in the magnetotail current sheet. The equations of motion can be expressed in terms of drift parameters which vary slowly and smoothly at the drift rate, not at the gyration rate. The analysis leads to boundaries in phase space which form a generalized loss cone and separate particles drifting into and out of the layer from particles trapped within the layer. These boundaries can be used in the moment integrals for densities and currents when the drifting particles differ in temperature, or in other properties, from the trapped population, as has been suggested by observations. We give examples of how different kinds of particle orbits contribute to the spatial profiles of density and current and thus to the field structure of the current sheet. We find that the parallel pressure of the drifting particles must exceed the transverse pressure for self-consistent solutions to exist, and based on this result, we give examples of fully self-consistent solutions using bi-Maxwellian ion and Maxwellian electron distributions. We give a proof, using generalized adiabatic theory, of Cowleys (1978a) theorem that particles trapped in the current layer experience zero net drift.

A three-ring circuit model of the magnetosphere

We have modeled the magnetosphere by superimposing a dipole field, a uniform field and a perturbation field due to a simple current system. This current system consists of a ring current in the neutral line of the dipole plus uniform fields, together with vertical currents representing field-aligned currents to the neutral line. The current circuit is closed by two additional ring currents above and below the equatorial plane representing distributed adiabatic perpendicular currents. This system produces many magnetospheric features including a magnetopause, bending of magnetic field lines in the anti-solar direction, a magnetotail, and cusps on the day-side of the Earth. Our aim is to demonstrate that it is not necessary to think of the magnetic field topology as being caused by the flowing plasma carrying field lines. The fundamental physical problem is to derive the current system from the self-consistent interaction of the solar-wind and magnetospheric plasmas and fields.

Experiments on regulation of electric charge on space vehicles

Spacecraft at geosynchronous altitudes have been observed to charge to potentials of many kilovolts. Anomalous behavior of spacecraft systems are believed to have resulted from discharges associated with these charging events. Experiments in modifying spacecraft charge have been conducted with ion and electron emitters on the ATS-5, ATS-6 and SCATHA spacecraft. The experiments have been successful in discharging highly charged spacecraft, in reducing the amount of differential charging on spacecraft surfaces, and in inducing charging on otherwise uncharged or nominally charged spacecraft. Regulation of vehicle charge allows better measurements of the plasma environment and should reduce anomalous spacecraft behavior.