The diffusion of astrophysical magnetic fields in conducting fluids in the presence of turbulence depends on whether magnetic fields can change their topology or reconnect in highly conducting media. Recent progress in understanding fast magnetic reconnection in the presence of turbulence is reassuring that the magnetic field behavior in computer simulations and turbulent astrophysical environments is similar, as far as the magnetic reconnection is concerned. This makes it meaningful to perform MHD simulations of turbulent flows in order to understand the diffusion of magnetic field in astrophysical environments. These simulations support the concept of reconnection diffusion, which describes the ability of magnetic fields to get removed from magnetized clouds and cores in the process of star formation.
A collision-induced magnetic reconnection (CMR) mechanism was recently proposed to explain the formation of a filament in the Orion A molecular cloud. In this mechanism, a collision between two clouds with antiparallel magnetic fields produces a dense filament due to the magnetic tension of the reconnected fields. The filament contains fiber-like sub-structures and is confined by a helical magnetic field. To show whether the dense filament is capable of forming stars, we use the \textsc{Arepo} code with sink particles to model star formation following the formation of the CMR-filament. First, …
The rate of magnetic field diffusion plays an essential role in several astrophysical plasma processes. It has been demonstrated that the omnipresent turbulence in astrophysical media induces fast magnetic reconnection, which consequently leads to large-scale magnetic flux diffusion at a rate independent of the plasma microphysics. This process is called ``reconnection diffusion'' (RD) and allows for the diffusion of fields which are dynamically important. The current theory describing RD is based on incompressible magnetohydrodynamic (MHD) turbulence. In this work, we have tested quantitative…
A unique filament is identified in the {\it Herschel} maps of the Orion A giant molecular cloud. The filament, which, we name the Stick, is ruler-straight and at an early evolutionary stage. Transverse position-velocity diagrams show two velocity components closing in on the Stick. The filament shows consecutive rings/forks in C$^{18}$O(1-0) channel maps, which is reminiscent of structures generated by magnetic reconnection. We propose that the Stick formed via collision-induced magnetic reconnection (CMR). We use the magnetohydrodynamics (MHD) code Athena++ to simulate the collision between t…
Disc formation in turbulent cloud cores: is magnetic flux loss necessary to stop the magnetic braking catastrophe or not?
Santos-Lima、de Gouveia Dal Pino和Lazarian（2012年）最近对湍流磁化云核中开普勒盘形成的数值分析表明，重联扩散是一个有效的过程，可以在旋转支撑盘的形成过程中去除多余的磁通量。这个过程是由湍流中磁场的快速重联引起的。在一项类似的数值研究中，Seifried等人（2012年）得出结论，重联扩散或任何其他非理想MHD效应都是不必要的，仅湍流剪切就可以提供一种自然的方式来建立旋转盘，而不需要磁通量损失。他们的结论是基于这样一个事实，即在半径远大于圆盘的球形区域上计算的平均质量通量比（μ）在他们的模型中几乎是恒定的。在这封信中，我们比较了两组模拟，并表明这种在大尺度上的平均可以掩盖在圆盘形成的内部区域μ的显着实际增加。我们表明，在吸积的湍流包络材料中，在盘形成的初始阶段发生了磁场的重联扩散。我们的分析是暗示，重连扩散是存在于这两套模拟，并提供了一个简单的解决方案的“磁制动灾难”，这是在文献中讨论的形成原恒星吸积盘。
Recently, a collision-induced magnetic reconnection (CMR) mechanism was proposed to explain a dense filament formation in the Orion A giant molecular cloud. A natural question is that whether CMR works elsewhere in the Galaxy. As an initial attempt to answer the question, this paper investigates the triggering of CMR and the production of dense gas in a flat-rotating disk with a modified BiSymmetric Spiral (BSS) magnetic field. Cloud-cloud collisions at field reversals in the disk are modeled with the Athena++ code. Under the condition that is representative of the warm neutral medium, the clo…
Modeling collisionless magnetic reconnection rate is an outstanding challenge in basic plasma physics research. While the seemingly universal rate of an order $\mathcal{O}(0.1)$ is often reported in the low-$β$ regime, it is not clear how reconnection rate scales with a higher plasma $β$. Due to the complexity of the pressure tensor, the available reconnection rate model is limited to the low plasma-$β$ regime, where the thermal pressure is arguably negligible. However, the thermal pressure effect becomes important when $β\gtrsim \mathcal{O}(1)$. Using first-principle kinetic simulations, we s…
