We have clarified the structure of asymmetric magnetic reconnection in detail as the result of the spontaneous evolutionary process. The asymmetry is imposed as ratio k of the magnetic field strength in both sides of the initial current sheet (CS) in the isothermal equilibrium. The MHD simulation is carried out by the HLLD code for the long-term temporal evolution with very high spatial resolution. The resultant structure is drastically different from the symmetric case (e.g., the Petschek model) even for slight asymmetry k = 2. (1) The velocity distribution in the reconnection jet clearly shows a two-layered structure, i.e., the high-speed sub-layer in which the flow is almost field aligned and the acceleration sub-layer. (2) Higher beta side (HBS) plasma is caught in a lower beta side plasmoid. This suggests a new plasma mixing process in the reconnection events. (3) A new large strong fast shock in front of the plasmoid forms in the HBS. This can be a new particle acceleration site in the reconnection system. These critical properties that have not been reported in previous works suggest that we contribute to a better and more detailed knowledge of the reconnection of the standard model for the symmetric magnetic reconnection system.
The properties of the sheared/guide field magnetic reconnection (MRX) are investigated with two-dimensional MHD simulation. We simulate the spontaneous evolution from the isothermal current sheet (CS) equilibrium in which distribution of the thermodynamical quantities is symmetric about the CS. The magnetic shear is characterized by two parameters: the shear parameter and the asymmetry parameter. The asymmetry of the Alfvén speed (V A0x) perpendicular to the X-line along the CS is essential. We focus on the asymptotic self-similarly expanding phase of the evolution. This research is unique for the discussion based on the consistency across the entire MRX system, although the sheared MRX has been studied since the early 1980s. In addition to reconfirmation of the previously reported properties of the sheared MRX, the following new properties are found. (1) The reconnection jet changes to the “core–envelope structure” (a high-density core with a low-density envelope) for the sheared symmetric V A0x case but the “two-layered structure” (the high-speed, low-density layer and the medium-speed, high-density layer) for the asymmetric V A0x case. (2) The parameter dependence of the reconnection rate is clarified. The MRX is fastest for the symmetric case and slows as the asymmetry increases for any fixed shear angle. For the symmetric case, the reconnection rate has a monotonically decreasing dependence on the shear angle. (3) In the asymmetric case, the plasmas from both sides of the CS coexist on the same magnetic field lines in the larger V A0x side plasmoid. This characteristic structure suggests an efficient plasma mixing when the plasmoid breaks.
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.
The behavior of extremely asymmetric magnetic reconnections is numerically investigated. The asymmetry is defined as the ratio k of the magnetic fields on both sides of the isothermal initial current sheet. This work is an extension of our previous research for 1 < k ≤ 2 to further asymmetry 2 < k ≤ 20. In our previous work, Nitta et al., we clarified that even for a slight asymmetry k ≤ 2, the reconnection structure drastically changes from symmetric standard models like the Petschek model. The properties of the asymmetric system are a (1) two-layered non-uniform reconnection jet, (2) contact discontinuity (CD) in the lower beta side (LBS) plasmoid between the plasmas coming from both sides of the current sheet, and (3) forward fast shock (FFS) in front of the higher beta side (HBS) plasmoid. We aim to clarify, in this paper, how these properties change and whether new aspects appear for further asymmetric cases. We have confirmed that, even under strongly asymmetric circumstances, the CD in the LBS plasmoid and the two-layered jet structure hold; however, the FFS disappears for extremely asymmetric cases. The fraction of the HBS plasma component increases in the reconnection outflow as k increases. The reconnection rate decreases as a power-law function of k.
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