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  • 1
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    A numerical code for a three-dimensional magnetospheric MHD equilibrium model (1992)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: Two dimensional and three dimensional MHD equilibrium models were begun for Earth's magnetosphere. The original proposal was motivated by realizing that global, purely data based models of Earth's magnetosphere are inadequate for studying the underlying plasma physical principles according to which the magnetosphere evolves on the quasi-static convection time scale. Complex numerical grid generation schemes were established for a 3-D Poisson solver, and a robust Grad-Shafranov solver was coded for high beta MHD equilibria. Thus, the effects were calculated of both the magnetopause geometry and boundary conditions on the magnetotail current distribution.
    Keywords: GEOPHYSICS
    Type: NASA-CR-189852 , NAS 1.26:189852
    Format: application/pdf
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  • 2
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    On the configuration of the polar cusps in earth's magnetosphere (1985)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: The interaction between the solar wind and the earth's vacuum dipole field leads to the formation of a discontinuity called the magnetopause. In the standard picture, the magnetopause confines the magnetic field in such a manner that the polar cusp field lines originate from high latitudes in the dayside ionosphere and end at the two magnetic neutral points. Wu (1983, 1984) has questioned this standard picture of the polar cusp. MHD simulations indicate the existence of a current sheet above the polar cusp region, called 'the cusp current sheet' by Wu. Wu (1983) concluded that the difference between his cusp configuration and the standard picture is due to the fact that his geometry results from a plasma model, whereas the standard picture is based on a vacuum concept. In the present investigation, Wu's conclusion is questioned, and it is demonstrated that the standard cusp configuration is not restricted to the vacuum magnetosphere.
    Keywords: GEOPHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 90; 4046-405
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  • 3
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    Computer simulation of inner magnetospheric dynamics for the magnetic storm of July 29, 1977 (1982)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: The Rice University convection model is applied to the early main phase of the July 29, 1977 magnetic storm through a computer implementation that self-consistently calculates electric fields, currents, and plasma distributions and velocities in the inner magnetosphere/ionosphere system. On the basis of solar wind parameters and AL index as inputs, the model predicts the injection of plasma sheet plasma to form a substantial storm time ring current whose total predicted strength agrees with the observed Dst index. The possibility that the magnetic field may be sufficiently inflated to make 60 deg field lines extend to the outer magnetosphere is examined. In the model, distortion of the plasma sheet inner edge by magnetospheric compression associated with the sudden commencement temporarily disrupts the normal Birkeland current pattern. Normal Birkeland currents and shielding reassert themselves in about an hour.
    Keywords: GEOPHYSICS
    Type: Journal of Geophysical Research; 87; Aug. 1
    Format: text
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  • 4
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    Magnetospheric equilibrium configurations and slow adiabatic convection (1986)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: This review paper demonstrates how the magnetohydrostatic equilibrium (MHE) theory can be used to describe the large-scale magnetic field configuration of the magnetosphere and its time evolution under the influence of magnetospheric convection. The equilibrium problem is reviewed, and levels of B-field modelling are examined for vacuum models, quasi-static equilibrium models, and MHD models. Results from two-dimensional MHE theory as they apply to the Grad-Shafranov equation, linear equilibria, the asymptotic theory, magnetospheric convection and the substorm mechanism, and plasma anisotropies are addressed. Results from three-dimensional MHE theory are considered as they apply to an intermediate analytical magnetospheric model, magnetotail configurations, and magnetopause boundary conditions and the influence of the IMF.
    Keywords: GEOPHYSICS
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  • 5
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    Solution of the Chapman-Ferraro problem with an arbitrary magnetopause (1994)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: We present a global model of the magnetic field of the magnetosphere that includes the effects of the Chapman-Ferraro currents at the magnetopause. In contrast to ealier models, the magnetopause shape is arbitrary, thus allowing the use of more realistic geometries. The internal magnetospheric field model of Hilmer and Voigt (1993), is completely shielded within the magnetopause by solving the Laplace equation with Neumann boundary conditions using a finite difference method on a non-orthogonal, curvilinear grid. The resulting model magnetosphere is perfectly closed although the method can also be applied with more general boundary conditions, to generate a set of open models based on the approach of Toffoletto and Hill (1989, 1993). The purpose of this paper is to demonstrate the feasibility of a purely numerical approach to solving the Chapman-Ferraro problem with arbitrary magnetopause shape and boundary conditions.
    Keywords: GEOPHYSICS
    Type: Geophysical Research Letters (ISSN 0094-8276); 21; 7; p. 621-624
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  • 6
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    Steady state magnetic field configurations for the earth's magnetotail (1989)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: A two-dimensional, force-balance magnetic field model is presented. The theoretical existence of a steady state magnetic field configuration that is force-balanced and consistent with slow, lossless, adiabatic, earthward convection within the limit of the ideal MHD is demonstrated. A numerical solution is obtained for a two-dimensional magnetosphere with a rectangular magnetopause and nonflaring tail. The results are consistent with the convection time sequences reported by Erickson (1985).
    Keywords: GEOPHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 94; 1303-131
    Format: text
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  • 7
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    Quasi-static magnetospheric MHD processes and the 'ground state' of the magnetosphere (1988)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: The possible existence of a theoretically well-defined ground state of the earth's magnetosphere is discussed in the context of magnetic equilibrium and convection theories. The quasi-static MHD theory is reviewed and a two-dimensional model is used to show that the convecting magnetosphere can reach a steady state. It is suggested that, under the influence of convection, magnetic substorms occur periodically in the magnetosphere and are an integral part of the entire convection cycle. The concept of the ground state of the magnetosphere is examined. The term 'average magnetosphere', is proposed for defining a baseline configuration that corresponds to average solar wind conditions.
    Keywords: GEOPHYSICS
    Type: Reviews of Geophysics (ISSN 8755-1209); 26; 823-843
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  • 8
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    A note on two-dimensional asymptotic magnetotail equilibria (1994)
    In:  Other Sources
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    Publication Date: 2019-08-28
    Description: In order to understand, on the fluid level, the structure, the time evolution, and the stability of current sheets, such as the magnetotail plasma sheet in Earth's magnetosphere, one has to consider magnetic field configurations that are in magnetohydrodynamic (MHD) force equilibrium. Any reasonable MHD current sheet model has to be two-dimensional, at least in an asymptotic sense (B(sub z)/B (sub x)) = epsilon much less than 1. The necessary two-dimensionality is described by a rather arbitrary function f(x). We utilize the free function f(x) to construct two-dimensional magnetotail equilibria are 'equivalent' to current sheets in empirical three-dimensional models. We obtain a class of asymptotic magnetotail equilibria ordered with respect to the magnetic disturbance index Kp. For low Kp values the two-dimensional MHD equilibria reflect some of the realistic, observation-based, aspects of three-dimensional models. For high Kp values the three-dimensional models do not fit the asymptotic MHD equlibria, which is indicative of their inconsistency with the assumed pressure function. This, in turn, implies that high magnetic activity levels of the real magnetosphere might be ruled by thermodynamic conditions different from local thermodynamic equilibrium.
    Keywords: GEOPHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 99; A4; p. 5943-5948
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  • 9
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    A magnetospheric magnetic field model with flexible current systems driven by independent physical parameters (1995)
    In:  Other Sources
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    Publication Date: 2019-08-27
    Description: A tilt-dependent magnetic field model of the Earth's magnetosphere with variable magnetopause standoff distance is presented. Flexible analytic representations for the ring and cross-tail currents, each composed of the elements derived from the Tsyganenko and Usmanov (1982) model, are combined with the fully shielded vacuum dipole configurations of Voigt (1981). Although the current sheet does not warp in the y-z plane, changes in the shape and position of the neutral sheet with dipole tilt are consistent with both MHD equilibrium theory and observations. In addition, there is good agreement with observed Delta B profiles and the average equatorial contours of magnetic field magnitude. While the dipole field is rigorously shielded within the defined magnetopause, the ring and cross-tails currents are not similarly confined, consequently, the model's region of validity is limited to the inner magnetosphere. The model depends on four independent external parameters. We present a simple but limited method of simulating several substorm related magnetic field changes associated with the disrupion of the near-Earth cross-tail current sheet and collapse of the midnight magnetotail field region. This feature further facilitates the generation of magnetic field configuration time sequences useful in plasma convection simulations of real magnetospheric events.
    Keywords: GEOPHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 100; A4; p. 5613-5626
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  • 10
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    A study of weak anisotropy in electron pressure in the tail current sheet (1995)
    In:  Other Sources
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    Publication Date: 2019-08-27
    Description: We adopt a magnetotail model with stretched field lines where ion motions are generally nonadiabatic and where it is assumed that the pressure anisotropy resides only in the electron pressure tensor. We show that the magnetic field lines with p(perpendicular) greater than p(parallel) are less stretched than the corresponding field lines in the isotropic model. For p(parallel) greater than p(perpendicular), the magnetic field lines become more and more stretched as the anisotropy approaches the marginal firehose limit, p(parallel) = p(perpendicular) + B(exp 2)/mu(sub 0). We also show that the tail current density is highly enhanced at the firehose limit, a situation that might be subject to a microscopic instability. However, we emphasize that the enhancement in the current density is notable only near the center of the tail current sheet (z = 0). Thus it remains unclear whether any microscopic instability can significantly alter the global magnetic field configuration of the tail. By comparing the radius of the field-line curvature at z = 0 with the particle's gyroradius, we suspect that even the conventional adiabatic description of electrons may become questionable very close to the marginal firehose limit.
    Keywords: GEOPHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 100; A3; p. 3573-3584
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