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Center for Nonlinear Studies |
Magnetic reconnection can change the topology of magnetic fields in a highly conducting plasma. It plays a key role in magnetic field relaxation in many astrophysical and laboratory plasmas, from solar and stellar flares to magnetic confinement devices. To explain many such phenomena, it is important to understand how the rates of magnetic reconnection behave in large and weakly collisional systems. A key question concerns what physics must be retained within reduced models to be able to reproduce the reconnection rates and global behaviour of fully kinetic systems. Here we consider the effect of the guide field, a normal and non-reconnecting component of the magnetic field, on the local reconnection physics and global evolution of the reconnecting system. It is demonstrated for weak guide field that the commonly used Hall-MHD fluid model is unable to reproduce the reconnection rate, pile-up field, outflow velocity or diffusion region geometry of fully kinetic simulations. Instead, a hybrid model with kinetic ions and fluid electrons is the minimum sufficient model to reproduce these key features of the problem [1]. For the strong guide field regime we evaluate a very simple reduced two-fluid model against cold ion fully kinetic simulations. Good agreement is found in both the rate and overall length of the layer, despite visible differences in electron scale physics [2]. References: [1] A. Stanier, W. Daughton, L. Chacon, H. Karimabadi, J. Ng, Y.-M. Huang, A. Hakim, and A. Bhattacharjee, Phys. Rev. Lett. 115, 175004 (2015). [2] A. Stanier, A. N. Simakov, L. Chacon, and W. Daughton, Phys. Plasmas 22, 101203 (2015).