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  • 1
    Electronic Resource
    Electronic Resource
    Computer studies on the spontaneous fast reconnection evolution in a force-free current sheet system (2002)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 9 (2002), S. 816-823 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: 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 β. Once an anomalous resistivity is ignited and a sufficient amount of the sheared field component Bz 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 Bz 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. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1447257
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  • 2
    Electronic Resource
    Electronic Resource
    Computer studies on the spontaneous fast reconnection evolution in various physical situations (2001)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 8 (2001), S. 1545-1552 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The spontaneous fast reconnection evolution is studied in a long current sheet system in various physical situations, where the threshold of current-driven anomalous resistivity is assumed to increase with the thermal velocity. If the initial threshold VC0 is sufficiently large in a low-β plasma, the fast reconnection mechanism can fully be set up; on the other hand, if VC0 is so small that the anomalous resistivity can easily occur in the usual circumstances, the resulting diffusion region notably lengthens so that the reconnection process becomes much less effective. Also, the fast reconnection evolution is strongly influenced by plasma β in the ambient magnetic field region, and an essential condition for the fast reconnection mechanism to evolve explosively is that the plasma β is sufficiently small. In fact, only in a low-β plasma does the magnetic tension force involved play the dominant role in the overall system dynamics and in the drastic magnetic energy release. It is also demonstrated that the fast reconnection evolution does not depend on the detailed functional form of the (current-driven) anomalous resistivity model. This is because the positive feedback between the anomalous resistivity and the reconnection flow effectively works so long as an anomalous resistivity is assumed to increase with the relative electron-ion drift velocity. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1360212
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  • 3
    Electronic Resource
    Electronic Resource
    Computer studies on the spontaneous fast reconnection model as a nonlinear instability (1999)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 6 (1999), S. 1522-1531 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The spontaneous fast reconnection model is examined for a current-driven anomalous resistivity model where the threshold for resistivity occurrence is assumed to increase with the temperature (thermal velocity). It is demonstrated that the fast reconnection mechanism can effectively evolve even if the threshold notably increases with the temperature increase near an X neutral point. The evolutionary process can be characterized by two distinct phases. In the initial phase, coupled to the anomalous resistivity, the reconnection process grows rather slowly. In the subsequent explosive phase, as soon as the plasma and the reconnected magnetic flux are effectively ejected from near the X point, it drastically grows due to the powerful positive feedback between the localized enhancement of anomalous resistivity and the growth of reconnection flow. For the larger resistivity threshold, the localization of anomalous resistivity near the X point becomes more effective, so that the fast reconnection development becomes more drastic. On the nonlinear saturation level, the (quasi-steady) fast reconnection mechanism is set up, where the peak fast reconnection rate is sustained, and standing slow shocks, attached to the localized diffusion region, are extended outwards with time. On the other hand, for uniform resistivity, the explosive phase does not occur, so that the fast reconnection mechanism cannot build up. Hence, it is concluded that the spontaneous fast reconnection model describes a new-type nonlinear instability in a long current sheet system, leading to drastic collapse of the overall magnetic field system. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.873405
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  • 4
    Electronic Resource
    Electronic Resource
    Computer studies on powerful magnetic energy conversion by the spontaneous fast reconnection mechanism (1995)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 2 (1995), S. 388-397 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: On the basis of the spontaneous reconnection model, computer simulations study the physical mechanism by which magnetic energy, initially stored in a current sheet system, is released into plasma energies. For the uniform resistivity model, the Sweet–Parker mechanism is eventually set up with the diffusion region becoming longer with time. It is the Ohmic heating ηJ2 in the diffusion region that plays the dominant role in releasing the magnetic energy. Attached to the diffusion region, a long plasmoid is formed and propagates like a large-amplitude Alfvén pulse, where the generator and motor effects are canceled along the plasmoid boundary. For the anomalous resistivity model, the fast reconnection mechanism is eventually set up with the diffusion region remaining to be localized near an X neutral point. It is the powerful motor effect [u⋅(J×B)(approximately-greater-than)0] along the slow shock layers that drastically releases the stored magnetic energy. A large-scale plasmoid distinctly swells, so that the ambient magnetic fields are compressed (by the generator effect), and the enhanced magnetic energy is then reduced by the strong motor effect in the backward half of the plasmoid. The slow shocks extend with time from near the X point, leading to a drastic catastrophe for the overall magnetic field system. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.870965
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  • 5
    Electronic Resource
    Electronic Resource
    Adiabatic expansion acceleration mechanism of superfast jets in the spontaneous fast magnetic reconnection model (2000)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 7 (2000), S. 2417-2424 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: 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 Alfvén 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 Alfvén 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 Alfvén 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. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.874080
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  • 6
    Electronic Resource
    Electronic Resource
    Computer studies on the spontaneous fast reconnection mechanism in three dimensions (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 853-862 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: 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 (Alfvénic) 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. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871789
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  • 7
    Electronic Resource
    Electronic Resource
    Computer studies on noncoplanar slow and intermediate shocks associated with the sheared fast reconnection mechanism (1994)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 1 (1994), S. 296-307 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: 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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.870831
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  • 8
    Electronic Resource
    Electronic Resource
    Computer simulations of asymmetric spontaneous fast reconnection (2000)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 7 (2000), S. 867-874 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The spontaneous fast reconnection model is extended to the general asymmetric situations, where magnetic reconnection may take place between the field lines that are anchored in different magnetic dipole sources. It is demonstrated for a wide variety of parameters that the asymmetric fast reconnection mechanism can evolve as a nonlinear instability, so that an asymmetric plasmoid swells predominantly in the region of a weaker magnetic field and propagates along the field lines. In the central diffusion region, the secondary tearing is likely to take place and significant erosion of the stronger magnetic field occurs; accordingly, the X neutral point moves with time, where the (current-driven) anomalous resistivity is found to be always locally enhanced, allowing the fast reconnection mechanism to be sustained quasisteadily and extended outwards further. The associated shock structure standing at the boundary of the stronger magnetic field is identified with the ordinary slow shock, in general, combined with an intermediate wave in the presence of sheared field. On the other hand, the shock standing at the boundary of the weaker magnetic field has an intermediate shock-like structure near the diffusion region, which propagates along the shock layer to finally become the ordinary combination of a slow shock and a finite-amplitude rotational intermediate wave at the plasmoid boundary. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.873883
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  • 9
    Electronic Resource
    Electronic Resource
    Computer studies on dynamics of a large-scale magnetic loop by the spontaneous fast reconnection model (1996)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 4172-4180 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The temporal dynamics of a large-scale magnetic loop is numerically studied on the basis of the two-dimensional spontaneous fast reconnection model. When a plasmoid, caused by the fast reconnection, propagates and collides with a wall boundary, across which plasma cannot flow, a large-scale magnetic loop is formed. The resulting magnetic loop is constructed by the reconnected field lines; inside the loop, the plasma, initially residing in the current sheet, is confined. As the reconnected field lines are piled up, the magnetic loop grows and swells outwards, so that a strong fast shock suddenly builds up at the interface between the growing loop and the strong reconnection jet. The fast shock, located ahead of the loop top, moves outwards with the growing loop, changing its strength with several peak and bottom Mach numbers. Accordingly, a localized spot-like region, where the plasma pressure is extremely enhanced, definitely comes out immediately ahead of the loop top. Along the loop side boundary, slow shocks stand, so that the resulting large-scale magnetic loop provides a very powerful energy converter in the sense that it is enclosed by slow and fast shocks. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871549
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  • 10
    Electronic Resource
    Electronic Resource
    Computer studies on plasmoid dynamics associated with the spontaneous fast reconnection mechanism (1995)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 2 (1995), S. 3320-3328 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The spontaneous fast reconnection model is applied to the plasmoid dynamics, and computer simulations are performed for a wide range of parameters. According to the fast reconnection development, the resulting strong (Alfvénic) plasma jet drives a large-scale plasmoid to swell and propagate. Once the plasmoid fully develops, the propagation speed Vp becomes almost constant. It is the J×B force, not the pressure-gradient force, that plays the dominant role on the plasmoid propagation. It is estimated that Vp∼0.8VA l0, where VA l0 is the Alfvén velocity measured in the magnetic field region ahead of the plasmoid. Along the backward-half plasmoid boundary a strong slow (almost switch-off) shock is formed, whereas along the forward-half boundary a rather weak slow shock stands. The plasma pressure is largely enhanced in the middle of the plasmoid, and the magnetic energy just outside the plasmoid is notably enhanced in the forward half because of the generator effect and is then distinctly reduced in the backward half by the strong motor effect. For the uniform resistivity model, any distinct plasmoid cannot be set up. It is argued that the spontaneous fast reconnection mechanism should be most applicable to catastrophic events observed in space plasmas. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.871165
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