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At least 631 records · Page 35
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NASA NTRS1985

MHD analysis of the evolution of solar magnetic fields and currents in an active region

A self-consistent, newly developed, nonplanar MHD model is used to investigate the evolution of field and current structures in solar active regions due to photospheric shearing motions. It is demonstrated that this model can predict physical parameters that may help improve understanding of the evolution of those field and current structures. Numerical results on the magnetic energy intensity, electric current intensity, and magnetic field configuration are presented.

Wu, S. T.↗
NASA NTRS1985

An MHD simulation of By-dependent magnetospheric convection and field-aligned currents during northward IMF

A three-dimensional MHD simulation code is used to model the magnetospheric configuration when the IMF has both a northward B(z) component and a B(y) component in the east-west direction. Projections of the plasma pressure, the field-aligned velocity, the field-aligned vorticity, and the field-aligned current along the magnetic field lines into the northern ionosphere are shown and discussed. Cross-sectional patterns of these parameters are shown. The results demonstrate that the B(y) component of the IMF strongly influences the plasma sheet configuration and the magnetospheric convection pattern.

Ogino, T.↗
NASA NTRS1985

MHD modelling of the earth's magnetosphere

Attention is given to the shock capturing technique, nonuniform grid system, and multiple time scale problem aspects of a global MHD model of the earth's magnetosphere that uses numerical methods. Because of the fast Alfven waves near the earth, the model is more difficult to solve than problems of hydrodynamic flow past bodies. The Rusanov (1962) scheme, which is a variant of the Lax (1954) method that maintains viscosity at a minimum value except where a large one is required, is used. The magnetosphere shape obtained by these means is noted.

Wu, C. C.↗
NASA NTRS1986

The phase relationship between gyrophase-bunched ions and MHD-like waves

Recent calculations and computer simulations of the right-hand resonant ion beam instability have indicated that gyrophase-bunched ion distributions may be generated as a first step in the disruption of field-aligned beams. Theory predicts a distinct phase angle relationship between the gyrovelocity of the phase-bunched ions and the wave magnetic field. In this paper, observations of such a relationship between gyrophase-bunched ions and MHD-like waves upstream from the earth's bow shock are presented. The observed phases are in accord with the theoretical predictions of the right-hand resonant ion beam instability.

Fuselier, S. A.↗
NASA NTRS1986

The motion of ions specularly reflected off a quasi-parallel shock in the presence of large-amplitude, monochromatic MHD waves

A model is used to examine the motion of specularly reflected ions in the presence of large-amplitude, monochromatic, transverse MHD waves. The calculations of ion trajectories are described. The heating downstream from the quasi-parallel bow shock is analyzed. The relationship between the specularly reflected ions and their gyrospeeds and guiding center speeds is studied. The data reveal that the characteristics of the motion depend on the frequency, wavelength, phase, and amplitude of the wave that is converted into the shock.

Fuselier, S. A.↗
NASA NTRS1986

A three-dimensional MHD simulation of the interaction of the solar wind with the earth's magnetosphere - The generation of field-aligned currents

The time-dependent interaction of the solar wind with the earth's magnetosphere is simulated using a three-dimensional MHD model. The bow shock, magnetopause, magnetotail, and plasma sheet of the magnetosphere and Birkeland field-aligned currents that are dependent on the polarity of the z component of the IMF are produced. Twin convection cells and a dawn to dusk electric potential of 30-100 kV are detected at the equator in the magnetosphere. Four types of field-aligned currents are observed: region 1, region 2, dayside magnetopause currents in the dayside cusp region, and the dayside cusp currents for southward IMF. Region 1 and 2 field-aligned currents generated for all IMF conditions are 0.6-1.0 x 10 to the 6th A and 0.15-0.61 x 10 to the 6th A, respectively. The relationship between region 1 currents and field-aligned vorticity, and region 2 currents and pressure gradients are studied. The simulated data are compared with a theoretical analysis of the field-aligned currents and good correlation is observed.

Ogino, T.↗
NASA NTRS1986

Cosmic-ray shock acceleration in oblique MHD shocks

A one-dimensional, steady-state hydrodynamical model of cosmic-ray acceleration at oblique MHD shocks is presented. Upstream of the shock the incoming thermal plasma is subject to the adverse pressure gradient of the accelerated particles, the J x B force, as well as the thermal gas pressure gradient. The efficiency of the acceleration of cosmic-rays at the shock as a function of the upstream magnetic field obliquity and upstream plasma beta is investigated. Astrophysical applications of the results are briefly discussed.

Webb, G. M.↗
NASA NTRS1986

Three-dimensional, time-dependent, MHD model of a solar flare-generated interplanetary shock wave

A three-dimensional time-dependent MHD model of the propagation of an interplanetary shock wave into an ambient three-dimensional heliospheric solar wind is initialized with a peak velocity of 1000 km/s at the center of a right circular cone of 18 deg included angle at 18 solar radii. Differences from a previous 2-1/2 simulation (Wu et al., 1983; Gislason et al., 1984; Dryer et al., 1984) include diminuation of the solar peak velocity and concentration of the peak density at each radius. The IMF magnitude starts with high-latitude peaks, and helical-like IMF rotation is noted due to a large-amplitude nonlinear Alfven wave in the shocked plasma.

Dryer, M.↗
NASA NTRS1986

An MHD simulation of the effects of the interplanetary magnetic field By component on the interaction of the solar wind with the earth's magnetosphere during southward interplanetary magnetic field

The interaction between the solar wind and the earth's magnetosphere has been studied by using a time-dependent three-dimensional MHD model in which the IMF pointed in several directions between dawnward and southward. When the IMF is dawnward, the dayside cusp and the tail lobes shift toward the morningside in the northern magnetosphere. The plasma sheet rotates toward the north on the dawnside of the tail and toward the south on the duskside. For an increasing southward IMF component, the plasma sheet becomes thinner and subsequently wavy because of patchy or localized tail reconnection. At the same time, the tail field-aligned currents have a filamentary layered structure. When projected onto the northern polar cap, the filamentary field-aligned currents are located in the same area as the region 1 currents, with a pattern similar to that associated with auroral surges. Magnetic reconnection also occurs on the dayside magnetopause for southward IMF.

Ogino, T.↗
NASA NTRS1986

An MHD simulation of the interaction of the solar wind with the outflowing plasma from a comet

The interaction between the solar wind and the outflowing plasmas from a comet has been studied by using a two-dimensional time-dependent magnetohydrodynamic (MHD) simulation. The model reproduced several features of the comet-solar wind interaction predicted by earlier theories and observed on the recent cometary probes. These include the formation of the contact surface and the cometary magnetotail. For a constant interplanetary magnetic field (IMF) the cometary plasma captures field lines which drape over the comet to form an antiparallel magnetic field configuration in the tail and a thin plasma sheet. Eventually, tail magnetic reconnection begins to occur at several points. When the IMF orientation is reversed dayside magnetic reconnection occurs at the subsolar point and a large disturbance propagation down the tail.

Ogino, T.↗
NASA NTRS1986

Field-aligned plasma flow in MHD simulations of magnetotail reconnection and the formation of boundary layers

The outflow from a reconnection region in a realistic magnetotail geometry is studied using a two-dimensional time-dependent, compressible, resistive MHD code. Two cases are emphasized: (1) the evolution of near-earth reconnection, which grows in the form of an internal unstable mode after gradual externally forced changes have initiated an anomalous dissipation process, and (2) the evolution of more distant reconnection under influence of a nonuniform inflow that forces reconnection to occur at a given location in the distant tail. In both cases, it is demonstrated that plasma flow is primarily parallel to the magnetic field in regions away from the localized area of reconnection and outside a narrow central layer.

Birn, J.↗
NASA NTRS1986

MHD simulation of the 'geoeffectiveness' of interplanetary disturbances

A series of examples is used to evaluate the 'geoeffectiveness' prediction technique that is based on the classical, initial boundary value problem of MHD. For the nonplanar simulation of a corotating stream, it is shown that the technique can compute relevant solar wind parameters at the earth's location. For the nonplanar simulation of a complex series of events, qualitative agreement is found for large-scale structures, but the phasing and amplitudes are not satisfactory. Preliminary work with a fully three-dimensional, time-dependent simulation of a flare-generated interplanetary shock wave shows that the effect of the latitudinal variation produces attenuation of the heliolongitudinal component of the IMF and of the radial velocity.

Dryer, M.↗
NASA NTRS1987

A linear MHD instability analysis of solar mass ejections with gravitation

The linear MHD instability of a cylindrical plasma is used to investigate the origin of solar mass ejections, and the dispersion relation is solved numerically. The initial plasma-flow velocity is found to have a significant effect on the instability criteria and growth rate, and the instability growth-rate is shown to be larger in cases where plasma flow exists, relative to the static case. Results suggest that the plasma column may break into small pieces. Assuming a thin-tube approximation, gravity is found to have little effect on the instability of quasi-horizontal ejection, but to have considerable effect on the vertical ejection. In considering the gravitational force, an exact analytical solution is found for the vertical case, while asymptotic solutions are given for the horizontal and oblique cases.

Song, M. T.↗
NASA NTRS1987

Cometary MHD and chemistry: Application to Halley

An MHD and chemical comet coma model was applied to the plasma flow, the magnetic field, and the ion abundances in Comet Halley. By alternating iterations between axisymmetric models with detailed chemistry and three dimensional models with rigorous account of the Lorentz forces, a consistent description of the plasma flow and chemical evolution in the ionized cometary coma is derived. Models appropriate to Comet Halley in which the magnetized plasma flow, the bow shock, the magnetic cavity of size 5000 km, and the model ion abundances at 1500 km and 6000 km from the nucleus are in qualitative agreement with the Giotto data were derived. The axisymmetric model correctly shows the three groupings of ions at each distance. The model abundances for the light ions, up to 21 amu, are in very good agreement with the 1500 km observations. The comparison becomes worse at higher molecular masses and greater distances from the nucleus.

Wegmann, R.↗
NASA NTRS1986

MHD bending waves in a current sheet

Transverse MHD bending waves are considered in an isothermal and compressible two-dimensional current sheet of finite thickness in which the magnetic field changes direction and strength. The general form of the wave equation is obtained. It is shown that rotation of the magnetic field across the current sheet prevents the existence of singular points so that continuous spectrum solutions and the concomitant wave decay disappear. Instead, normal modes exist and closed integral solution for arbitrary current sheet structure are found. The results are discussed in terms of small-scale waves on the heliospheric current sheet.

Musielak, Z. E.↗
NASA NTRS1987

MHD wave breaking in the outer plasmasphere

Empirical models of the average magnetospheric magnetic field, plasma density, and temperature distributions are used to construct a model of the distribution of MHD wave mode speeds within the magnetosphere. A persistent feature of the derived optical structure is a pronounced minimum of the wave speeds in the outer plasmasphere, i.e., a magnetospheric 'shoal'. This feature does not map along magnetic field lines, but is confined to the equatorial region, leading to a positive radial gradient of wave speeds near synchronous orbit. The breaking of earthward propagating disturbances in this region may play an essential role in the formation of the substorm injection boundary and in the creation of equatorially trapped warm ion distributions.

Moore, T. E.↗
NASA NTRS1988

Employing analogies for ducted MHD waves in dense coronal structures

Analogies of fast MHD waves propagating along a dense coronal structure are exploited to examine how the size and shape of the inhomogeneity affect the properties of the Love- and Pekeris-type waves. The profile's shape determines the dispersive nature of the waves. Excited impulsively, magnetic Love and Pekeris waves give rise to quasi-periodic oscillations with a duration and time scale that depend on the cross-sectional area and strength of the inhomogeneity. More diffuse coronal inhomogeneities support impulsively generated periodic oscillations, with the quasi-periodic signature being absent.

Edwin, P. M.↗
NASA NTRS1987

Numerical tests of a modified full implicit continuous Eulerian (FICE) scheme with projected normal characteristic boundary conditions for MHD flows

A numerical method has been developed based on a modified full implicit continuous Eulerian (FICE) scheme and projected normal characteristic boundary conditions for simulating MHD flows which undergo a long process of evolution. An astrophysical flow is chosen for illustration of this procedure, and numerical tests are made to verify the computational stability and physically realistic solution. Three computational tests have been accomplished; they are tests of solving methods, characteristic boundary condition, and time steps. The tests show that the program from the modified FICE scheme with proper boundary conditions and time steps can be made numerically stable for a time long enough to obtain physically plausible solutions.

Wu, S. T.↗