Fundamental Properties of Sheared/Guide Field MHD Magnetic Reconnection

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 (VA0x) 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 VA0x case but the "two-layered structure" (the high-speed, low-density layer and the medium-speed, high-density layer) for the asymmetric VA0x 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 VA0x side plasmoid. This characteristic structure suggests an efficient plasma mixing when the plasmoid breaks.