太阳物理研究组知识库

The effects of the initial upstream plasma beta on the plasmoid instability are studied via two-dimensional resistive magnetohydrodynamic simulations. For cases with nonuniform beta dependent initial plasma mass density and uniform temperature, our numerical results indicate that the critical Lundquist number for onset of the plasmoid instability depends on the initial plasma beta. The critical Lundquist number is approximately 2000 - 3000 for beta = 50 and is 8000 - 10000 for beta = 0.2. The higher the beta, the smaller the critical Lundquist number is. Similar to previous studies of high-beta systems, the average reconnection rate in low beta systems is found to become weakly dependent on the Lundquist number in the plasmoid-unstable regime. However, the average reconnection rate, normalized to the asymptotic value of upstream BVA, is lower in a low beta system than that in a high beta system. The magnetic energy spectral index, which characterizes fragmentation of the reconnection layer, is approximately two and is insensitive to beta in the high-Lunquist number regime. It is also found that the magnetic reconnection rate becomes similar for different beta cases, if the initial force-balance is provided by temperature gradient instead of density gradient. Therefore, it is concluded that the beta-dependence mentioned above may be largely attributed to the density variation.