Engineering topics

Birn, Joachim

Publications and source records attributed to Birn, Joachim.

29 records · Page 2

NASA NTRS1992

MHD modeling of magnetotail instability for anisotropic pressure

Results of three-dimensional MHD simulations of magnetotail dynamics with anisotropic pressure are presented. The pressure tensor is assumed to be gyrotropic, satisfying a modified double adiabatic approximation including Ohmic heating. It is found that these constraints tend to stabilize the tail with respect to the resistive tearing instability. Including an increasing level of additional isotropization results in a gradual transition to the fast instability found in an isotropic model, which is discussed for comparison. Possible isotropization mechanisms yielding magnetotail instability are discussed also. The stabilizing anisotropies are found primarily in the plasma sheet boundary region. This fact indicates a possibly important role of isotropization mechanisms operating in this region in destabilizing the magnetotail and initiating a tearing instability without the necessity of changes in fluctuation levels in the neutral sheet.

Hesse, Michael↗

NASA NTRS1992

Magnetotail instability

The stability of the geomagnetic tail is investigated on the basis of three dimensional resistive magnetohydrodynamic simulations, using different dynamic constraints and different initial equilibria. Different forms of the energy equation for isotropic pressure are found to have no significant effect on the dynamic growth of a resistive tearing instability, which is responsible for near Earth reconnection, plasmoid formation and ejection, and the generation of fast plasma flows. The constraints of a modified double adiabatic approach, however, can quench the tearing instability through the development of large, mirror type, anisotropies in the boundary regions of the plasma sheet, unless isotropization occurs on fast, nearly Alfvenic, time scales. The presence of a net cross tail magnetic field component B(sub yN) can reduce the growth of the instability without complete stabilization. An increase of B(sub z) from midnight toward the tail flanks, however, by more than a factor of about 3, apparently completely stabilizes the tearing mode. Stabilization and destabilization thus may depend on properties and constraints (and their release) in regions other than the neutral sheet where reconnection is initiated.

Birn, Joachim↗

NASA NTRS1992

Relative timing of substorm features in MHD simulations

An investigation of the temporal sequence of substorm phenomena based on three dimensional MHD (magnetohydrodynamic) simulations of magnetic reconnection and plasmoid formation is presented. The investigation utilizes a spatially localized resistivity model which leads to a significantly faster evolution than found in previous investigations. The analysis of the results concentrates on substorm features that have received considerable attention in the past. The formation of magnetic neutral lines, the occurrence of fast flows directed both earthward and tailward, and the magnetic field changes leading to the formation of the substorm current wedge, and to the depolarization of the magnetic field earthward of the reconnection region and its dependence on the spatial distribution of resistivity, are discussed. These phenomena are seen as an integral part of the nonlinear evolution of the three dimensional tearing instability.

Hesse, Michael↗

NASA NTRS1992

Three-dimensional MHD modeling of magnetotail dynamics for different polytropic indices

The paper analyzes the 3D resistive tearing instability in a magnetotail configuration by means of a resistive MHD code for various forms of the energy equation, including ohmic and compressional heating. A complete derivation of the set of MHD equations from a kinetic formalism is presented, with all simplifying assumptions noted. Emphasis is placed on the changes in magnetotail evolution caused by a variation of the assumed value of the polytropic exponent gamma, representing the ratio of specific heat in the energy equation which closes the set of MHD equations. Within the gamma range of 2/3-4, a tearing mode is found to develop in all cases. Certain properties, such as the formation and ejection of a plasmoid, associated with the formation of the reversal region of the north-south component of the magnetic field, and strong earthward and tailward flow fields prevail as qualitative features independent of the value of gamma chosen. Ohmic dissipation is found to be the major source of the heating, while adiabatic or nonadiabatic compression may contribute primarily in the closed field region earthward of the reconnection site.

Hesse, Michael↗

NASA NTRS1991

On dipolarization and its relation to the substorm current wedge

The near-earth properties of magnetospheric substorms are investigated using the Hesse and Birn (1991) self-consistent 3D MHD model of magnetotail activity. The simulation shows that an increase of Bz, a decrease of the cross-tail current, and an increase of region-1 field-aligned currents occurs roughly simultaneously, consistent with the empirical picture described by McPherron et al. (1973). It was also found that the region where the cross-tail current is reduced and diverted is located tailward from the dipolarization region. The results indicate that the apparent tailward propagation of the dipolarization effects does not support an earlier suggestion that the underlying process is initiated further earthward.

Hesse, Michael↗

NASA NTRS1991

The boundary value problem of magnetotail equilibrium

The earth magnetotail equilibrium problem is examined by expressing it as a 2D boundary value problem for a prescribed tail boundary, which may be considered as an idealized magnetopause. A general solution of this problem is derived for the 2D case, where the dependence on the y coordinate and the presence of By are neglected. These solutions are further generalized by including the y dependence and an open magnetosphere. Then, a general solution of a 3D boundary problem is derived.

Birn, Joachim↗

NASA NTRS1991

Magnetosphere-ionosphere coupling during plasmoid evolution - First results

The influence of magnetosphere-ionosphere coupling on the dynamic evolution of the magnetotail is investigated by a three-dimensional resistive MHD code that includes the effects of the closure of field-aligned currents in a simple resistive model ionosphere. Particular emphasis is on the role of this coupling during substorm evolution and the modification of the latter by the convection driven by the ionospheric electric fields. For comparison, results are presented from a simulation which uses an infinitely conducting ionosphere but is otherwise identical. Comparison of the two simulations shows that the major impact of magnetosphere-ionospheric communication is an acceleration of magnetotail evolution. Otherwise, phenomena in the two models are qualitatively similar. It is concluded that ionospheric effects do not significantly affect substorm associated magnetotail dynamics.

Hesse, Michael↗

NASA NTRS1991

The geomagnetic tail

A review is presented of the plasma sheet and lobe regions of the magnetotail, focusing principally on large-scale processes or microprocesses with some large-scale effects. Consideration is given to quiet and average structures, not necessarily related to activity phases, with quasi-steady convection aspects, and with the characteristics of dynamic phases including acceleration mechanisms and single particle aspects. Attention is given to various activity models, average and quiet time properties, properties and effects of magnetospheric convection, dynamics of the magnetotail, and the near tail, substorm current wedge.

Birn, Joachim↗

NASA NTRS1990

A self-consistent two-dimensional resistive fluid theory of field-aligned potential structures including charge separation and magnetic and velocity shear

A self-consistent two-fluid theory that includes the magnetic field and shear patterns is developed to model stationary electrostatic structures with field-aligned potential drops. Shear flow is also included in the theory since this seems to be a prominent feature of the structures of interest. In addition, Ohmic dissipation, a Hall term, and pressure gradients in a generalized Ohm's law, modified for cases without quasi-neutrality, are included. In the analytic theory, the electrostatic force is balanced by field-aligned pressure gradients (i.e., thermal effects in the direction of the magnetic field) and by pressure gradients and magnetic stresses in the perpendicular direction. Within this theory, simple examples of applications are presented to demonstrate the kind of solutions resulting from the model. The results show how the effects of charge separation and shear in the magnetic field and the velocity can be combined to form self-consistent structures such as are found to exist above the aurora, suggested also in association with solar flares.

Hesse, Michael↗

NASA NTRS1990

On the topology of flux transfer events

A topological analysis is made of a simple model magnetic field of a perturbation at the magnetopause that shares magnetic properties with flux transfer events. The aim is to clarify a number of topological aspects that arise in the case of fully three-dimensional magnetic fields. It is shown that a localized perturbation at the magnetopause can in principle open a closed magnetosphere by establishing magnetic connections across the magnetopause by the formation of a ropelike magnetic field structure. For this purpose a global topological model of a closed magnetosphere is considered as the unperturbed state. The topological substructure of the model flux rope is discussed in detail.

Hesse, Michael↗

NASA NTRS1989

Parallel electric fields in a simulation of magnetotail reconnection and plasmoid evolution

Properties of the electric field component parallel to the magnetic field (E sub parallel) in a three-dimensional MHD simulation of plasmoid formation and evolution in the magnetotail in the presence of a net dawn-dusk magnetic field component were observed. Particularly emphasized was the spatial location of E(sub parallel), the concept of a diffusion zone and the role of E(sub parallel) in accelerating electrons. A localization of the region of enhanced E(sub parallel) in all space directions with a strong concentration in the z direction was found. This region was identified as the diffusion zone, which plays a crucial role in reconnection theory through the local break-down of magnetic flux conservation. The presence of B(sub y) implies a north-south asymmetry of the injection of accelerated particles into the near-earth region, if the net B(sub y) field is strong enough to force particles to follow field lines through the diffusion region. It is estimated that for a typical net B(sub y) field this should affect the injection of electrons into the near-earth dawn region, so that precipitation into the Northern (Southern) Hemisphere should dominate for duskward (dawnward) net B(sub y). In addition, a spatial clottiness of the expected injection of adiabatic particles which could be related to the appearance bright spots in auroras was observed.

Hesse, Michael↗