ISSN:0148-0227    

DOI:10.1029/92JA02313

年代:1992

数据来源: WILEY

摘要:

The ion tearing instability is investigated in a magnetotail configuration that consists of a diffuse plasma sheet current and an embedded, thin current sheet with a strong current. For historical reasons, the thin embedded current sheet will be called a “neutral sheet”, even though the normal component of the magnetic field,Bn, is nonzero. In particular, we assume that the current within the thin current sheet is due to the acceleration of “Speiserlike” ion trajectories by a cross‐tail electric fieldEy. It is found that the strong current within the neutral sheet is essentially unimportant to the growth rate of the tearing instability, and that the growth rate scales as (λ0/Lz)², whereLzis the overall half thickness of the plasma sheet, λ0is the ion inertial length,c/ωpi, and ωpiis the ion plasma frequency at the neutral sheet edge. In the absence of the current outside the neutral sheet, current filamenta

ISSN:0148-0227    

DOI:10.1029/92JA01523

年代:1992

数据来源: WILEY

摘要:

The classical Petschek model of fast, steady state reconnection has been generalized in two families of reconnection regimes. The first family, which we refer to as “almost uniform,” models the reconnection of nearly uniform, antiparallel magnetic fields, and it includes Petschek's model as a special case. The second family, which we refer to as nonuniform, models the reconnection of strongly curved magnetic fields, and it includes separatrix jets and reversed current spikes at the ends of the diffusion region. In general, both families contain regimes having fast reconnection rates, but we show here that these fast reconnection regimes do not occur when the boundary conditions often used in numerical experiments are adopted. In 1986, D. Biskamp carried out a series of numerical experiments to check Petschek's prediction that the maximum reconnection rate should scale with the magnetic Reynolds number,Rme, as [ln (Rme)]−1. Biskamp found that the maximum reconnection rate in his experiments did not scale in this way but instead asRme−1/2. Because this corresponds to the scaling predicted by the slow reconnection theory of Sweet (1958) and Parker (1957), Biskamp has argued that his numerical experiments show that fast reconnection does not exist at high magnetic Reynolds numbers. However, by applying boundary conditions similar to Biskamp's to the “nonuniform” family of reconnection regimes, we are able to explain Biskamp's scaling results and to explain why he did not achieve fast reconnection in his numerical experiments. Therefore, we conclude that numerical experiments with suitably designed boundary conditions are highly likely to exhibit fast reconnection and that such reconnection is a common process in astrophysical and sp

ISSN:0148-0227    

DOI:10.1029/92JA01146

年代:1992

数据来源: WILEY

摘要:

The structure and stability of thin one‐dimensional current sheets in which the ions carry most of the current and momentum are investigated. It is demonstrated that for the case of a cold current sheet the characteristic half width λ is given by λ ≈ (Bz/B0)4/3c/ωp0, whereBzis the normal field component (assumed to be spatially uniform),B0is the asymptotic magnitude of the reversing field, andc/ωp0is the collisionless ion skin depth based on the asymptotic density. The trapping of the ion orbits in the reversal region leads to a strong density enhancement at the center of the current sheet given bync/n0≈c/λωp0≈ (B0/Bz)4/3. A two‐dimensional self‐consistent dynamical simulation model is developed which demonstrates that these idealized current sheets are unstable to kink perturbations driven by the anisotropic pressure distribution produced by the chaotic nature of the particle orbits in a field‐reversal region. The combined effects of chaotic scattering and finite thermal spread are likely to preclude the formation in the geomagnetic tail of stable current sheets with half width less than about

ISSN:0148-0227    

DOI:10.1029/92JA01550

年代:1992

数据来源: WILEY

摘要:

Approximate equilibrium solutions for the two‐dimensional magnetohydrodynamic equilibrium equations with flow along magnetic field lines applied to modeling the Earth's magnetic tail are constructed. An alternative version of the condition for whether a given magnetospheric configuration is open or closed is presented. Also developed are examples of open and closed magnetospheric configurations with flow occurring in a layer modeling the plasma sheet boundary layer. Examples of finite‐size compressible plasmoids traveling in the magnetotail are presented. The relation between the profile of the total pressure (thermal plus magnetic) and the shape of the plasmoid is also discus

ISSN:0148-0227    

DOI:10.1029/92JA01432

年代:1992

数据来源: WILEY

摘要:

A 2 1/2‐D nonradiative, electromagnetic particle code is used to investigate processes in the plasma sheet in the Earth's magnetotail that can generate the field‐aligned currents associated with auroral arcs. The setting of the simulation is the midnight meridian plane on closed magnetic field lines. The magnetic field is supported self‐consistently by a plasma sheet population. The system is driven by an external convection electric field that causes particles from the lobe region to drift onto the plasma sheet. Momentum exchange between the field and the lobe particles accelerated through the plasma sheet balances the sunward Maxwell stresses. The particle and field boundaries are spatially separated by a collisional layer representing the effect of the neutral atmosphere. During the simulation, new particles are continually injected into the system from the “ionospheric” regions. The simulations show the generation of currents circulating in the noon‐midnight plane due to the differing degrees of magnetization between ions and electrons. These currents give rise to magnetic fields perpendicular to the noon‐midnight meridian plane. North‐south asymmetries in the lobe plasma density enhance current circulation between the plasma sheet and the ionospheric boundaries. Necessary conditions for generation of auroral currents suggest a new interpretation of the growth and expansive phases of the a

ISSN:0148-0227    

DOI:10.1029/92JA01542

年代:1992

数据来源: WILEY

摘要:

General properties of field‐aligned plasma flow in quasi‐steady ideal MHD configurations are discussed, and explicit solutions, modeling compressible flow around a plasmoid in the distant magnetotail, are presented. The explicit solutions are based on the assumption that plasma and field gradients along thexandydirections parallel to the tail current sheet are typically much smaller than those in thezdirection perpendicular to the current sheet. In contrast to the incompressible case, two different flow solutions exist in the compressible case in the subalfvénic regime. The different regimes are distinguished by whether the flow speed is below or above a critical speed υo, which represents the group velocity of a slow magnetosonic wave (aligned with the magnetic field) in the limit of phase propagation perpendicular to the magnetic field, where the phase speed vanishes. In the incompressible case, only the subcritical flow solution is realized in the subalfvénic regime. In the compressible case, both regimes may exist simultaneously in the flow around the plasmoid. They are separated by a tangential discontinuity which may be considered as the limit of a slow shock for vanishing normal flow and normal magnetic field. The subcritical flow region surrounding the plasmoid inside of a supercritical region possibly corresponds to the layers of tailward streaming ions observed adjacent to the plasmoid proper. The signatures of a passage from the supercritical through the subcritical flow region in the vicinity of the plasmoid can be similar to those of a passage through the plasmoid itself: a reduction ofBxfollowed by a local enhancement in the middle of the encounter, as observed, for instance, by Hones et al. (1984) and Slavin et al. (1989). This enhancement is found for a simple loop structure within the plasmoid and hence need not be associated with multiple loops or a tail flapping as suggested by Hones et al. Field signatures outside the plasmoid are typically those of encounters of a traveling compression region: an enhancement ofBxaccompanying a north‐south signature ofBz. In certain parameter regimes, however, unusual signatures are also possible: a reduction ofBxwith a north‐southBzsignature for a close plasmoid encounter (without penetration) and a south‐north signature ofBzaccompanying aBxenhancement for a distant plasmoid encounter at hi

ISSN:0148-0227    

DOI:10.1029/92JA01527

年代:1992

数据来源: WILEY

摘要:

In space plasmas, e.g., planetary magnetospheres and the solar wind, it has been observed that particle velocity distributions typically possess a non‐Maxwellian high‐energy tail that can be well modeled by a generalized Lorentzian (kappa) distribution. The generalized Lorentzian distribution is characterized by a spectral index κ, varies as {energy}−(κ+1)at high velocities, and approaches a Maxwellian distribution as κ → ∞. As a natural analogue to the widely used plasma dispersion functionZ(ξ), which is based on the Maxwellian distribution, we have recently introduced a new special functionZκ*(ξ) based on the generalized Lorentzian distribution; we callZκ*(ξ) the modified plasma dispersion function. BecauseZκ*(ξ) can be expressed in simple closed form,Zκ*(ξ) is easier to use thanZ(ξ) both from analytical and computational points of view.Zκ*(ξ) is, moreover, a natural tool for analyzing microinstabilities in a variety of space plasmas. In this paper we useZκ*(ξ) to analyze three classical problems of plasma physics: Landau damping of Langmuir waves; ion acoustic instability in a current‐carrying plasma; and cyclotron resonant instability of electromagnetic R mode waves propagating parallel to an ambient magnetic field. In each case we find that results for a generalized Lorentzian plasma can differ significantly from those in a Maxwellian plasma. Previous calculations based on a Maxwellian distribution, that purport to apply to waves in space, may therefore

ISSN:0148-0227    

DOI:10.1029/92JA01664

年代:1992

数据来源: WILEY

摘要:

The dayside to nightside circulation of plasma along the magnetopause inside the magnetosphere is examined by means of three‐dimensional single‐particle codes. It is demonstrated that particles incident upon the outer cusp region experience transient non‐adiabatic motions, owing to a localized minimum in the field magnitude. Here, possibly large magnetic moment changes yield injection into the loss cone of fractions of the incoming population or, alternatively, enhanced bouncing motions at high altitudes. It is shown that particles gaining access to the magnetotail over the polar cap are progressively extracted from the weak field region by the large‐scale convection electric field. In this latter case, the trajectory simulations suggest an implicit “entry boundary” into the nightside magnetosphere, which corresponds to the sunward edge of field lines featuring monotonic decrease of the field magnitude along t

ISSN:0148-0227    

DOI:10.1029/92JA00834

年代:1992

数据来源: WILEY

摘要:

The magnetospheres of Earth and other planets, the solar wind, and the solar and stellar atmospheres are magnetically structured. The structuring can include tangential discontinuities. Wave propagation in magnetic structures consisting of an arbitrary number of parallel tangential discontinuities is considered. The governing equation for long nonlinear small‐amplitude waves is derived. The solutions of this equation in the form of algebraic solitary waves and nonlinear periodic waves are foun

ISSN:0148-0227    

DOI:10.1029/91JA02672

年代:1992

数据来源: WILEY