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At least 487 records · Page 27

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NASA NTRS1986

Turbulent magnetic reconnection

The results of two-dimensional spectral-method MHD simulations of the effect of turbulence on magnetic-reconnection processes are presented in extensive graphs and tables and characterized in detail. In a periodic sheet-pinch configuration, early-onset, persistent turbulence is found to occur when a low level of broadband fluctuations is introduced into the initial data. Features observed include small-scale unsteady coherent electric-current and vorticity structures, enhanced viscous and resistive dissipation, unsteady and spatially asymmetric fluid flow, large-scale and small-scale magnetic islands, multiple X points, large-amplitude electric-field fluctuations, and spectral characteristics indicating that the turbulence is broadband in both spatial directions and similar to homogeneous MHD turbulence. The importance of reconnection for studies of solar, magnetospheric, and laboratory plasmas in inhomogeneous magnetic fields is indicated.

Matthaeus, W. H.↗

NASA NTRS1987

Ion distributions in a two-dimensional reconnection field geometry

ISEE observations have shown trapped ion distributions in the magnetosphere along with streaming ion distributions in the magnetosheath. The more energetic ion beams are further away from the magnetopause than lower-energy ion beams. Predictions made with a simple two-dimensional reconnection model which contains a neutral line and an azimuthal electric field were compared with the experimental data of Sept., 1978. The model explains trapped particles in the magnetosphere due to nonadiabatic mirroring in the magnetosheath and streaming ions in the magnetosheath due to energization at the magnetopause. The model also shows the higher-energy ions extending further into the magnetosheath and farther away from the magnetopause than the lower-energy ions. This suggests the ion data of Sept., 1978 are consistent with a reconnection geometry.

Curran, D. B.↗

NASA NTRS1987

A magnetohydrodynamic simulation of reconnection in the magnetotail during intervals with southward interplanetary magnetic field

Results are reported from a simulation of the interaction between the solar wind and the earth magnetosphere, using a time-dependent three-dimensional MHD model. The calculation was performed for several orientations of the IMF between dawnward pointing and southward. When the IMF has a dawnward component, the plasma sheet rotates northward on the dawnside of the tail and toward the south on the duskside. As the southward component becomes larger, the plasma sheet becomes thinner and develops a wavy cross section because of patchy or localized tail reconnection. The field-aligned currents (FACs) associated with this localized reconnection have a filamentary layered structure. When projected onto the polar cap the filamentary FACs are located in the same region as the tail region 1 currents. At lower latitudes strong region 2 sense currents that originate in the plasma sheet are found. FACs are found on field lines that map to the polar cap even for southward IMF. These currents have many of the properties of the observed polar-cusp currents. The polar-cusp FACs evolve from the polar-cap NB(z) FACs as the IMF is rotated from northward to southward.

Walker, R. J.↗

NASA NTRS1988

Evidence for periodic reconnection at Uranus?

The unique orientation of Uranus at the time of the Voyager 2 encounter results in a convection dominated magnetosphere. Plasma and magnetic field data from the tail magnetosheath are presented. Velocity decreases of 5-10 percemt seem to occur with a 17-hour period. At least four repetitions of this decrease are observed, in all cases when flow passes over the dayside polar cap. One possible interpretation of these features is that they are signatures of dayside reconnection. The cause of the velocity decreases would be drag on the reconnected flux tubes which are coupled via Birkeland currents to the ionosphere. The coupling efficiency for power transfer between the solar wind and Uranian magnetosphere implied by these decreases is consistent with previous determinations of this quantity.

Richardson, J. D.↗

NASA NTRS1988

Mechanisms for fast flare reconnection

Normal collisional-resistivity mechanisms of magnetic reconnection have the drawback that they are too slow to explain the fast rise of solar flares. Two methods are examined which are proposed for the speed-up of the magnetic tearing instability: the anomalous enhancement of resistivity by the injection of MHD turbulence and the increase of Coulomb resistivity by radiative cooling. The results are described for nonlinear numerical simulations of these processes which show that the first does not provide the claimed effects, while the second yields impressive rates of reconnection, but low saturated energy outputs.

Vanhoven, G.↗

NASA NTRS1988

Does the resistive tearing instability nonlinearly evolve to a fast reconnection mode

A fundamental problem in applying linear tearing instability theory to the rapid processes (particle acceleration, heating) in flares was the characteristically slow rate of reconnection. This problem can be at least partially overcome if the tearin mode nonlinearly evolves to a regime in which the reconnection rate is substantially enhanced, such as that for the Petschek configuration. This possibility was often suggested, and some numerical simulations appear to provide support for such a view. Numerical simulation are used to study the nonlinear evolution of the tearing stability and show that a fast Petschek-like regime may not be achieved. This conclusion follows when there are sufficient grid points within the diffusion region to completely resolve the nonlinear dynamic interactions in the diffusion layer. When the numerical resolution is not adequate, the solution does appear to approach a Petschek configuration. The resolved solution contains reverse flow vortices and current sheets, terminated with a current reversal, similar to those obtained by Syrovatsky (JEPT, 33, 933, 1971).

Steinolfson, R. S.↗

NASA NTRS1989

Particle distributions in a two-dimensional reconnection field geometry

In order to understand space phase density variations near the magnetopause and in the geomagnetic tail, particle trajectories in a two-dimensional steady state reconnection field geometry are studied by tracing particles from a spacecraft position back in time. The distribution function at the spacecraft is calculated using Liouville's theorem and known distribution functions in the source regions. It is found, in particular, that multiple ion beams in the plasma sheet boundary layer are a natural consequence of reconnection. In the center of the central plasma sheet, the omnidirectional flux of keV ions should be at a minimum. The depth of this minimum is very large near the neutral point but decreases as the distance from the neutral point increases.

Gurran, D. B.↗

NASA NTRS1989

The formation flare loops by magnetic reconnection and chromospheric ablation

Noncoplanar compressible reconnection theory is combined here with simple scaling arguments for ablation and radiative cooling to predict average properties of hot and cool flare loops as a function of the coronal vector magnetic field. For a coronal field strength of 100 G, the temperature of the hot flare loops decreases from 1.2 x 10 to the 7th K to 4.0 x 10 to the 6th K as the component of the coronal magnetic field perpendicular to the plane of the loops increases from 0 percent to 86 percent of the total field. When the perpendicular component exceeds 86 percent of the total field or when the altitude of the reconnection site exceeds 10 to the 6th km, flare loops no longer occur. Shock-enhanced radiative cooling triggers the formation of cool H-alpha flare loops with predicted densities of roughly 10 to the 13th/cu cm, and a small gap of roughly 1000 km is predicted to exist between the footpoints of the cool flare loops and the inner edges of the flare ribbons.

Forbes, T. G.↗

NASA NTRS1989

Collisionless reconnection in a quasi-neutral sheet near marginal stability

Particle simulations are used to investigate the process of collisionless reconnection in a magnetotail configuration which includes a pressure gradient along the tail axis and tail flaring. In the absence of electron stabilization effects, the tearing mode is stabilized when the ion gyrofrequency in the normal field exceeds the growth rate in the corresponding one-dimensional current sheet. The presence of a low-frequency electromagnetic perturbation in the lobes can serve to destabilize a marginally stable current sheet by producing an extended neutral-sheet region which can then undergo reconnection. These results help to explain how X-type neutral lines, such as those associated with the onset of magnetospheric substorms, can be formed in the near-earth plasma sheet.

Pritchett, P. L.↗

NASA NTRS1991

Three-dimensional kinematic reconnection of plasmoids

The kinematic reconnection model of Lau and Finn (1990) is applied in a theoretical investigation of three-dimensional plasmoid morphology, as seen in the solar corona and earth magnetotail. The derivation of the governing equations is outlined; long, short, and periodic plasmoid models are developed; and the evolution of the stable and unstable manifolds in these models is presented graphically. It is inferred that sheet currents and tangential discontinuities can form on surfaces topologically identical to those where Delta(phi) about equal to Delta(z) singularities occur in the kinematic reconnection model, and can be broadened in a similar way by nonideal effects. Two such surfaces exist in long plasmoids, and also in short plasmoids in the presence of finite resistivity; the intertwined surfaces characteristic of the periodic plasmoid form a fractal set but merge in the presence of finite resistivity, producing structures similar to those proposed by the sheet-current theory of Parker (1983).

Lau, Yun-Tung↗

NASA NTRS1991

Dynamic magnetic reconnection at an X-type neutral point

The relaxation of a two-dimensional 'X-type' neutral point magnetic field disturbed from equilibrium is considered. Perturbations are shown to possess well-defined azimuthal modes which allow an exact determination of the magnetic annihilation rate. Free magnetic energy is dissipated by oscillatory reconnection which couples resistive diffusion at the neutral point to global advection of the outer field. The decay of azimuthally symmetric (m = 0) modes - the only modes associated with topological reconnection - is limited by the dissipation time scale of the 'fundamental' (n = 0) mode with no radial nodes. This mode decays over typically 100 Alfven times. An analytic treatment shows that the oscillation and decay time scales couple according to a given law.

Craig, I. J. D.↗

NASA NTRS1991

Reconnection voltage as a function of IMF clock angle

Magnetic reconnection between the IMF and the geomagnetic field is thought to play a major role in the transfer of solar wind momentum and energy to the magnetosphere. Both analytic modeling and analysis of geophysical data have shown that this coupling process should be a sensitive function of the clock angle of the IMF. Results are presented from a three-dimensional, MHD, global numerical simulation code for the reconnection voltage between the closed geomagnetic field and the IMF as a function of the IMF clock angle. These results are consistent with a sin(theta/2) functional behavior.

Fedder, J. A.↗

NASA NTRS1991

Structure of the dissipation region during magnetic reconnection in collisionless plasma

The results are presented of an analytic and numerical investigation of the structure of the X line during steady state magnetic reconnection in collisionless plasma. The structure of the X line essentially depends on a single dimensionless parameter F, which is a measure of the influx of plasma into the reconnection region. For small F the self-consistent plasma current driven at the X line is small, and the magnetic fields are nearly unchanged from the initial vacuum state. With increasing F the current driven at the X line becomes large and the dissipation region collapses in the direction of the inflow and elongates along the outflow. For sufficiently large F the velocity of the plasma ejected form the X line exceeds the local Alfven velocity. In this regime a fast mode shock forms at the outflow end of the dissipation region which slows the high-velocity outflow plasma to the subsonic flow characteristic of the broader outflow region. Finally, at a critical plasma flux the dissipation region collapses to zero thickness.

Burkhart, G. R.↗

NASA NTRS1991

Forced magnetic reconnection in a plasma sheet with localized resistivity profile excited by lower hybrid drift type instability

A forced magnetic reconnection process with a temporal evolution of resistivity is studied for a plasma sheet with a nonuniform resistivity profile based on the nonlocal mode structure of the lower hybrid drift type instability. The growth rate of the mode found is almost independent of the resistivity at the neutral sheet, but depends on the resistivity of the region of maximum density gradient away from the neutral sheet. This is studied by using both a nonlinear numerical MHD simulation and a linear theory. The mode may be relevant to the prevalent theoretical concept of MHD reconnection and the localized anomalous resistivity profile based on the lower hybrid drift instability.

Hoshino, M.↗

NASA NTRS1992

Magnetic flipping - Reconnection in three dimensions without null points

In three dimensions, magnetic reconnection may take place in a sheared magnetic field at any singular field line, where the nearby field has X-type topology in planes perpendicular to the field line and where an electric field is present parallel to the field line. In the ideal region around the singular line there will, in general, be singularities in the plasma flow and electric field, both at the singular line and at 'magnetic flipping layers', which are remnants of local magnetic separatrices. In the absence of a three-dimensional magnetic neutral point or null point, reconnection of field lines can still occur by a process of magnetic flipping, in which the plasma crosses the flipping layers but the field lines rapidly flip along them by magnetic diffusion. Depending on the boundary conditions, there may be two or four flipping layers which converge on the singular line. A boundary layer analysis of a flipping layer is given, in which the magnetic field parallel to the layer decreases as one crosses it while the plasma pressure (or magnetic pressure associated with the field along the singular line) increases. The width of the flipping layer decreases with distance from the singular line.

Priest, E. R.↗

NASA NTRS1992

The role of intermediate shocks in magnetic reconnection

The present study examines the structure of discontinuity layers associated with magnetic reconnection by numerically solving the Riemann problem for the evolution of an initial current sheet which separates two plasma regions with antiparallel magnetic field components in the z direction and a common guide magnetic field in the y direction. In the presence of a nonzero normal component of the magnetic field, the initial current sheet evolves into a system of MHD discontinuities. For the initial current sheet with a zero guide field, steady intermediate shocks, slow shocks, slow expansion waves, or contact discontinuity are observed to develop. For the current sheet with a nonzero guide field, time-dependent intermediate shocks, instead of steady intermediate shocks, are observed to bound the reconnection layer.

Lin, Y.↗

NASA NTRS1992

Magnetic blowout during collisionless reconnection

Analytic calculations and simulations of the steady-state structure of the dissipation region during collisionless reconnection are described. Ions are treated as particles and electrons as a dissipative fluid. A limiting reconnection rate results as the ions 'blowout' the magnetic field in the dissipation area. The implications for comprehending substorm dynamics are discussed.

Drake, J. F.↗

NASA NTRS1993

Comment on 'Observations of reconnection of interplanetary and lobe magnetic field lines at the high-latitude magnetopause' by J.T. Gosling, M.F. Thomsen, S.J. Bame, R.C. Elphic, and C.T. Russell

Comment is presented on the results of measurements, reported by Gosling et al. (1991), that were made on ISEE in the vicinity of the high-latitude dusk magnetopause near the terminator plane, at a time when the local magnetosheath and tail lobe magnetic fields were nearly oppositely directed. The character of the observed plasma flowing both tailward and sunward within the high-latitude magnetopause current layer presented real evidence for the local reconnection process. Gosling et al. argued that this process may be a manifestation of different global magnetospheric topology structures. In the comment, a global magnetospheric convection pattern is constructed for the northward IMF and for the case of a large azimuthal component of the IMF with small Bz, irrespective of its sign. The suggested scheme provides a simple explanation for the observed sunward convection in the polar caps both for the northward and for strong By with small Bz. According to the present model, for the magnetosheath field at 2300 UT on June 11, 1978, the reconnection between the open field lines appears at the northern neutral point.

Belen'kaia, Elena↗