Critical Differences of Asymmetric Magnetic Reconnection From Standard Models

Abstract

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.

Effects of Magnetic Shear and Thermodynamic Asymmetry on Spontaneous Magnetohydrodynamic Reconnection

“…Our previous studies on the self-similar MRX model (Nitta 2004(Nitta , 2006(Nitta , 2007Nitta et al 2001Nitta et al , 2002Nitta et al , 2016Nitta & Kondoh 2019, 2021 demonstrated slower MRX cases than the Petschek model (higher magnetic Reynolds number cases (Figure 4 of Nitta 2007), thermodynamically asymmetric cases about the CS (Figure 8 of Nitta & Kondoh 2019), and sheared magnetic field cases (Figures 10 and 11 of Nitta & Kondoh 2021)). In these cases, the reconnection rate significantly decreases from a maximum value (prediction of the Petschek model).…”

“…The prior works paid scant attention to this point. By considering the consistency across the entire region, the reconnection system can determine itself spontaneously" (Nitta et al 2016).…”

“…The numerical setup of the problem we adopted in this study is essentially the same as that in our previous papers (Nitta et al 2016;Nitta & Kondoh 2019, 2021; however, slight alterations had been made to simultaneously introduce the asymmetry of the thermodynamic quantities with regard to the CS and the magnetic shear. We consider mainly the isothermal CS equilibrium with the sheared field, in which the strength of the magnetic field and the distribution of thermodynamic quantities are asymmetric with regard to the CS.…”

“…As an additional investigation, the isodensity case (i.e., the temperature and pressure are asymmetric) is introduced. The degree of the thermodynamic asymmetry is represented by the ratio k of the magnetic field strength of the lower region (y < 0) to that of the upper region (y > 0) (Nitta et al 2016).…”

“…We present the generalized Newtonian (nonrelativistic) MRX model in this study and discuss the properties of the spatial structure of the MRX system and the parameter dependence of the reconnection rate. These were independently discussed in our recent studies for the thermodynamically asymmetric (Nitta et al 2016;Nitta & Kondoh 2019) or sheared cases (Nitta & Kondoh 2021). We determined new features of the sheared or thermodynamically asymmetric MRX compared to the shear-less symmetric MRX, for example the two-layered structure of the jet, the contact discontinuity (CD) formation in the plasmoid, the fast shock formation in front of the shorter plasmoid, and the decrease in the reconnection rate for the thermodynamically asymmetric or sheared situation.…”