Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

Three-dimensional instability of the spontaneous fast magnetic reconnection is studied with magnetohydrodynamic (MHD) simulation, where the two-dimensional model of the spontaneous fast magnetic reconnection is destabilized in three dimension. Generally, in two-dimensional magnetic reconnection models, every plasma condition is assumed to be uniform in the sheet current direction. In such two-dimensional MHD simulations, the current sheet destabilized by the initial resistive disturbance can be developed to fast magnetic reconnection by a current driven anomalous resistivity. In this paper, the initial resistive disturbance includes a small amount of fluctuations in the sheet current direction, i.e., along the magnetic neutral line. The other conditions are the same as that of previous two-dimensional MHD studies for fast magnetic reconnection. Accordingly, we may expect that approximately two-dimensional fast magnetic reconnection occurs in the MHD simulation. In fact, the fast magnetic reconnection activated on the first stage of the simulation is two dimensional. However, on the subsequent stages, it spontaneously becomes three dimensional and is strongly localized in the sheet current direction. The resulting three-dimensional fast magnetic reconnection intermittently ejects three-dimensional magnetic loops. Such intermittent ejections of the three-dimensional loops are similar to the intermittent downflows observed in the solar flares. The ejection of the three-dimensional loops seems to be random but, numerically and theoretically, it is shown that the aspect ratio of the ejected loops is limited under a criterion.

DOI： 10.1063/1.3095562

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：TERRA SCIENTIFIC PUBL CO  

Three-dimensional instability of spontaneous fast magnetic reconnection is studied using MHD (magneto-hydro-dynamic) Simulation. Previous two-dimensional MHD Studies have demonstrated that. if a current-driven anomalous resistivity is assumed, two-dimensional fast magnetic reconnection occurs and two-dimensional large- scale magnetic loops, i.e., plasmoids, are ejected from the reconnection re.-ion. In most two-dimensional MHD Studies, the Structure of the Current sheet is initially one-dimensinal. On the other hand, in recent space plasma observations, fully three-dimensional magnetic loops frequently appear even in the almost one-dimensional Current sheet. This suggests that the classical two-dimensional fast magnetic reconnection may be unstable to any three-dimensional perturbation, resulting in three-dimensional fast magnetic reconnection. In this paper, we show that a three-dimensional resistive perturbation destabilizes two-dimensional fast magnetic reconnection and results in three-dimensional fast magnetic reconnection. The resulting three-dimensional fast reconnection repeatedly ejects three-dimensional magnetic loops downstream. The obtained numerical results are similar to the pulsating downflows observed in solar flares. According to the Fourier analysis of the ejected magnetic loops, the time evolution of this three-dimensional instability is fully non-linear.

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The 3D spontaneous fast reconnection model is applied to well-known signatures of geomagnetic substorms and solar flares. First, it is applied to the traveling compression regions (TCRs) associated with plasmoids propagating down the tail plasma sheet, known its a definite signature of geomagnetic substorms. and the in-situ satellite observations can be precisely explained, both qualitatively and quantitatively. Then. it is demonstrated that the magnetospheric current wedge to link the fail Current,e drastically evolves through field-aligned currents to the auroral electrojet. It is also found that the well-known morphological features of two-ribbon flares call be explained by the fast reconnection model. In particular. the joule heating, associated with the flare Current wed-e. is shown to be important for the two-ribbon heating. Therefore, it is suggested that both solar flares and geomagnetic substorms result from the same physical mechanism, i.e., the fast reconnection mechanism.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE LTD  

The dynamics of large-scale magnetic loop in three dimensions is studied by MHD simulations. The spontaneous fast reconnection model is used in this study. Once a current-driven anomalous resistivity is ignited in a local region in a current sheet, the fast reconnection mechanism spontaneously evolves. As a result, large magnetic loop is developed, and very localized high pressure region appears outside of the magnetic loop. Near the region between magnetic loop and the high pressure region, very large vortex flow appears, and then, the high pressure region more and more localized due to this vortex. On the other hand, we suggest that the spatial size of initial disturbance to the direction of current is very important to evolve of three dimensional fast reconnection processes. (c) 2005 COSPAR. Published by Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.asr.2005.05.021

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE LTD  

MHD study for the adiabatic expansion acceleration process associated with the spontaneous fast magnetic reconnection is reported. When the fast reconnection process steadily generates a plasmoid in the downstream, the adiabatic expansion acceleration region appears between the reconnection jet and plasmoid. It is pointed out that the appearance of the acceleration region is required to steadily keep the reconnection process. The reconnection jet and plasmoid is generally high beta but the plasma pressure in the acceleration region is extremely low, when the reconnection jet is supersonic. This feature may become a signature to detect where the Petschek reconnection is steadily caused in the current sheet of the geomagnetotail. (c) 2005 COSPAR. Published by Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.asr.2005.05.119

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Language：English   Publishing type：Research paper (international conference proceedings)  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

The spontaneous fast reconnection model, is studied in a three-dimensional (3D) situation for different plasma parameter values. In any case, once a current-driven anomalous resistivity is ignited, magnetic reconnection explosively evolves as a nonlinear instability, and the 3D fast reconnection mechanism involving large-scale standing slow shocks is realized as an eventual solution on the nonlinear saturation phase. For the smaller plasma beta, the reconnection evolution is more drastic, and the resulting fast reconnection mechanism becomes more powerful. In the fast reconnection configuration, the central 3D diffusion region becomes unstable against resistive tearing and is bifurcated into a pair of diffusion regions, which move away from, each other. In the moving diffusion region, the locally enhanced anomalous resistivity is self-consistently sustained by the reconnection flow, and the slow shock stands between the 3D diffusion region and a large-scale 3D plasmoid. Since the plasmoid moves much more rapidly than the diffusion region, the 3D slow shock rapidly extends in the x direction in a finite extent in the z direction to occupy the overall system. In the wide range of plasma beta, the reconnection outflow jet u(x) attains the, Alfven velocity, measured in the ambient magnetic field region. Hence, the 3D fast reconnection mechanism established in the center of the system, which is consistent with the well-known 2D one, is sustained,steadily and extends outwards to drastically collapse the field system at large. (c) 2005 American Institute of Physics.

DOI： 10.1063/1.1883181

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DOI： 10.1016/S0964-2749(05)80022-8

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DOI： 10.1016/S0964-2749(05)80021-6

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

The present paper studies the basic physics of the spontaneous fast reconnection model in a three-dimensional (3D) situation for different resistivity parameter values, where the threshold for occurrence of current-driven anomalous resistivity is allowed to increase with the thermal velocity (rootT), and the initial plasma density notably changes in space with the plasma pressure in the current sheet system. For any case, once the anomalous resistivity is ignited, the 3D fast reconnection mechanism explosively evolves as a nonlinear instability by the positive feedback between the anomalous resistivity and the reconnection flow, even if the threshold significantly increases with the thermal velocity; for the larger threshold values, the fast reconnection evolution becomes more drastic and the reconnection rate, finally attained on the nonlinear saturation phase, becomes larger. In the resulting 3D fast reconnection configuration, slow shocks stand and extend outwards in the finite extent; also, ahead of the fast reconnection jet, a large-scale 3D plasmoid swells and propagates in the central current sheet, and a vortex flow is formed near the plasmoid side boundary. In the wide range of parameter values, the basic physics of the 3D spontaneous fast reconnection evolution in the finite extent is found to be, qualitatively, consistent with the well-known two-dimensional one. (C) 2004 American Institute of Physics.

DOI： 10.1063/1.1677110

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

The spontaneous fast reconnection evolution is studied in asymmetric magnetic field configuration. In particular, it is investigated how shear flow influences in magnetosheath region to the propagation of plasmoid results from magnetic reconnection using two-dimensional magnetohydrodynamic simulations. According to the fast reconnection development, the resulting large-scale plasmoids swell and propagate. Once the plasmoid fully develops, the propagation speed becomes almost constant in both the symmetric and asymmetric magnetic field configuration. An asymmetric plasmoid swells predominantly in the region of a weaker magnetic field and propagates along the field lines. The associated shock structure standing at the plasma boundary is the ordinary slow shock irrespective of the intensity of shear flow. However, velocity of plasmoid is proportional to shear flow velocity. (C) 2003 COSPAR. Published by Elsevier Ltd. All rights reserved.

DOI： 10.1016/S0273-1177(03)00644-6

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

Supersonic and subsonic expansion acceleration mechanisms associated with spontaneous fast magnetic reconnection process are compared by two-dimensional magnetohydrodynamic (MHD) simulations and test fluid (non-charged) particle simulations. When the Petschek reconnection process is steadily established, the reconnection jet generated by a pair of slow shocks becomes either supersonic (Case 1) or subsonic (Case 2), depending on the upstream plasma condition. For Case 1, the jet generated by the slow shocks can be further accelerated by the adiabatic supersonic expansion process. Finally, the jet encounters a fast shock in front of the plasmoid. For Case 2, the jet generated by slow shocks can be further accelerated by the adiabatic subsonic expansion process. The acceleration of Case I is stronger than that of Case 2. In Case 2, a fast shock is not formed. In both cases, it is important that the propagation of the plasmoid is driven by slow shocks formed around the plasmoid itself, rather than the reconnection jet. (C) 2003 COSPAR. Published by Elsevier Ltd. All rights reserved.

DOI： 10.1016/S0273-1177(03)00643-4

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The thermodynamic supersonic expansion acceleration mechanism associated with the spontaneous fast magnetic reconnection is studied by two-dimensional magnetohydrodynamic (MHD) simulations and the Rankine-Hugoniot analysis. The reconnection outflow jet can steadily exceed the Alfven velocity measured in the upstream magnetic field region. Such a high speed jet cannot be explained by the Petschek model. According to previous studies, when supersonic (superfast) plasma jets generated by a pair of slow shocks expand in the direction normal to the jet, the jets can be further accelerated beyond the Alfven velocity by the adiabatic supersonic expansion process. The expansion process is caused by the swelling of the plasmoid (magnetic loop). In this paper, it is theoretically shown that the sound Mach number of the reconnection jet generated by slow shocks is determined by the plasma density and beta value in the upstream magnetic field region, in which asymmetric reconnection models are also studied. Then, the theoretical prediction of the Mach number is related to the onset of the supersonic expansion acceleration process in MHD simulations. In addition, it is shown that, also when the reconnection jet is subsonic, the jet is further accelerated by the adiabatic subsonic expansion mechanism. (C) 2003 American Institute of Physics.

DOI： 10.1063/1.1555731

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

Three-dimensional (3D) dynamics of a large-scale magnetic loop is studied by precise magnetohydrodynamic simulations on the basis of the spontaneous fast reconnection model. Once a (current-driven) anomalous resistivity is ignited, the fast reconnection mechanism drastically evolves by the positive feedback between the (3D) global reconnection flow and the anomalous resistivity; on the nonlinear saturation phase, the global reconnection flow has grown so that the reconnection (diffusion) region shrinks to a small extent, and the fast reconnection mechanism involving a pair of standing slow shocks is established in the finite extent. When the 3D plasmoid, formed ahead of the fast reconnection jet, collides with the mirror plane boundary, the reconnected field lines are piled up, leading to formation of a large-scale 3D magnetic loop. Since the resulting 3D fast reconnection jet becomes supersonic, a definite fast shock builds up at the interface between the magnetic loop top and the fast reconnection jet. The 3D fast reconnection jet is limited in a narrow channel between the pair of slow shocks, so that the resulting fast shock is also limited to a small extent ahead of the magnetic loop top. On the other hand, for the uniform resistivity model the 3D fast reconnection mechanism cannot be realized without any vital positive feedback between the reconnection flow and the local magnetic diffusion; hence, such an effective resistivity that can be self-consistently enhanced locally at the X reconnection point by the global reconnection flow is essential for the fast reconnection mechanism to be realized in actual systems. (C) 2003 American Institute of Physics.

DOI： 10.1063/1.1536613

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DOI： 10.1016/S0273-1177(02)00025-X

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

Two-dimensional magnetohydrodynamic simulations study the spontaneous fast reconnection evolution in a force-free current sheet system where the magnetic field simply rotates by 180 deg across the central current sheet without changing its magnitude. It is demonstrated that, as in the conventional coplanar case, the fast reconnection mechanism drastically evolves because of the positive feedback between (current-driven) anomalous resistivity and global reconnection flow; also, the fast reconnection evolution becomes more drastic for the lower plasma beta. Once an anomalous resistivity is ignited and a sufficient amount of the sheared field component B-z is ejected from near the X reconnection point, the ambient magnetic field collapses into the X point, giving rise to the drastic buildup of the fast reconnection mechanism. On the nonlinear saturation phase, the B-z field is completely swept away from the reconnection region, so that coplanar slow shocks extend outward, and a large-scale plasmoid swells and propagates. The resulting plasmoid has a double structure that is quite different from the well-known coplanar one or from the so-called flux rope. In the backward half of the plasmoid, the plasma pressure is enhanced in the butterfly-shaped region, and (coplanar) slow shocks stand along the plasmoid boundary. On the other hand, in the forward half of the plasmoid a finite-amplitude intermediate wave stands along the plasmoid boundary; hence, across the plasmoid boundary, the magnetic field simply rotates without changing plasma quantities nor magnetic field magnitude. (C) 2002 American Institute of Physics.

DOI： 10.1063/1.1447257

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Language：Japanese   Publishing type：Research paper (scientific journal)  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：TERRA SCIENTIFIC PUBL CO  

Asymmetric spontaneous fast magnetic reconnection model, in which the current sheet is initially put between two antiparallel straight magnetic field regions with different magnetic intensities, is studied by two-dimensional magnetohydrodynamic (MHD) simulations. Especially, the supersonic expansion acceleration process which has been already observed in the symmetric case is focused on. In this paper, we find that if the initial asymmetry features are weak and one side of the magnetic field regions consists of sufficiently low beta plasma, the supersonic expansion acceleration process is generated in the reconnection jet region, like the symmetric case. Hence, in such an asymmetric case, a fast shock is formed at the top of the magnetic loop.

DOI： 10.1186/BF03353287

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In contrast to the Petschek reconnection model, the plasma outflow jet in front of the plasmoid associated with the spontaneous fast reconnection model is found to exceed steadily the Alfven velocity measured in the upstream magnetic field region. According to two-dimensional magnetohydrodynamic simulations, the final velocity of the plasma jet is observed to be superfast and can reach 1.4 times of the Alfven velocity, which is maintained until the jet encounters a fast shock generated in front of the plasmoid. On the basis of the Rankine Hugoniot relation and the Bernoulli equation, it is theoretically found that the superfast plasma jet generated by slow shocks associated with the reconnection process is effectively accelerated beyond the Alfven velocity by the adiabatic expansion of the plasma jet without any magnetic effect. In the plasma accelerations, the initial plasma acceleration caused in the slow shock is consistent with that of the Petschek reconnection model, but the subsequent plasma acceleration caused by the adiabatic expansion is not considered in his model. In association with the new acceleration mechanism, one pair of low-pressure regions emerges in the upstream magnetic field region. The generation of the low-pressure regions indicates that the significant adiabatic expansion results from the distortion of the surrounding magnetic field lines associated with the swelling plasmoid. (C) 2000 American Institute of Physics. [S1070-664X(00)04306-8].

DOI： 10.1063/1.874080

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：ASTRONOMICAL SOC PACIFIC  

On the basis of 2 dimensional MHD (Magnethydrodynamic) simulation, the plasma outflow jet, associated with the spontaneous fast reconnection model, is found to exceed steadily the Alfven velocity measured in the upstream magnetic field region. Such a high-speed plasma jet cannot be explained by the Petschek reconnection model. According to our theoretical study, such a high-speed plasma jet is generated by the following two acceleration stages. Firstly, superfast plasma jet is generated by the slow (switch off) shock associated with the fast reconnection process. This is consistent with the Petschek model. Second, the super fast plasma jet exceeds the Alfven velocity by a purely thermo-dynamic adiabatic expansion mechanism.

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：JAPAN SOCIETY PLASMA SCIENCE &NUCLEAR FUSION RESEARCH  

Acceleration processes of single test protons are studied in time-developing electric and magnetic fields that are obtained from two dimensional MHD simulations of the spontaneous fast magnetic reconnection. In this reconnection model, a current sheet is destabilized initially by small fluctuations. Then, a x-type magnetic field configuration spontaneously develops by a locally enhanced anomalous resistivity. The configuration includes a small magnetic diffusion region and a pair of strong slow shocks with alfvenic plasma jets; also a large-scale magnetic loop is formed by the reconnected magnetic field lines in the outflow region, and a fast shock emerges on the top of the magnetic loop.
It is shown that effective proton accelerations are performed in the three characteristic regions, generated by the reconnection mechanism; especially, particles, which are trapped in the magnetic loop top region with mirror motions, gain higher energy than the other particles. It is also shown that particle accelerations in the spontanous fast reconnection model are more abrupt and stronger than a tearing-type reconnection model, which proceeds in a uniform resistivity.

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The spontaneous evolution of fast reconnection is studied in three dimensions by extending (in the z direction) the previous two-dimensional model that considered only the x-y plane [M. Ugai, Phys. Fluids B 4, 2953 (1992)]. It is demonstrated that the reconnection development strongly depends on three-dimensional effects; only when the central current sheet is sufficiently long in the z direction, say more than a few times larger than the current sheet width, the fast reconnection mechanism fully develops by the self-consistent coupling between the global reconnection flow and the current-driven anomalous resistivity. In this case, the reconnection flow can grow so powerfully as to enhance the current density (the current-driven resistivity) locally near an X line; otherwise, such a vital reconnection flow cannot be caused. The resulting quasisteady fast reconnection mechanism is significantly confined in the z direction, where a strong (Alfvenic) plasma jet results from standing switch-off shocks; accordingly, a large-scale plasmoid is formed and propagates in the middle of the system. It is concluded that the well-known two-dimensional spontaneous fast reconnection model can reasonably be extended to three dimensions. (C) 1996 American Institute of Physics.

DOI： 10.1063/1.871789

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Language：Japanese   Publishing type：Doctoral thesis  

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society for Industrial and Applied Mathematics  

When charged particle trajectories are numerically calculated in discrete electro-magnetic fields obtained from computer simulations, it is necessary to interpolate spatially the field data. Usually, linear interpolation has been used for this purpose, but in this case adiabatic invariants are not conserved for the calculated particle trajectory. We hence propose an interpolation method, by which the solenoidal condition of magnetic field is exactly satisfied. The adiabatic invariants are conserved for particle trajectories calculated in the resulting continuous fields, and the use of the lowest order interpolation scheme demands cpu time only little more than the case of linear interpolation. This method is applied to two test magnetic fields of simple topology and a complicated electro-magnetic field data obtained from MHD simulation. It is argued that in order to calculate particle trajectories correctly magnetic field data should be interpolated so that the solenoidal condition is satisfied. It is also argued that this method is applicable to general plasma particle simulations.

DOI： 10.11540/jsiamt.5.4_423

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Language：Japanese   Publishing type：Research paper (scientific journal)  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

It was recently found that noncoplanar slow shocks stood in the sheared fast reconnection configuration. Hence, the present one-dimensional magnetohydrodynamics (MHD) simulations with high numerical resolution study the temporal dynamics of MHD shocks, from a slow shock to a weak intermediate shock, that are placed in a noncoplanar situation. It is shown that for any case the noncoplanar shock structure can be sustained by physical dissipations involved. The resulting noncoplanar slow shock structure is, both qualitatively and quantitatively, in good agreement with the two-dimensional shock transition layer associated with the sheared fast reconnection mechanism. The one-dimensional noncoplanar slow or (subfast) intermediate shock structure is eventually bifurcated into an intermediate wave and a coplanar slow shock as a result of magnetic field rotation. In general, any stable shock must be coplanar, and in actual systems strictly coplanar boundary conditions ahead of and behind a shock cannot be provided nor sustained. Hence we propose a criterion, required for a stable shock to be realized, such that the (coplanar) shock must survive and hence be derived as an eventual solution in noncoplanar situations. It is argued that the present simulation results as well as the previous ones should be interpreted and reconsidered on the basis of this criterion.

DOI： 10.1063/1.870831

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