太阳爆发大尺度磁重联过程的湍流性质研究

驱动太阳爆发的磁重联过程发生在位于日冕物质抛射和太阳耀斑之间的电流片当中，承担着90%以上的能量转换的任务，其中的物理细节和几何结构对这个过程起着至关重要的作用。理论与观测均表明该电流片具有非常大的尺度，其中的磁重联过程表现出明显的湍流特征，它是各种等离子体和磁流体动力学不稳定性（如撕裂模不稳定性）发展的结果，导致电流片内部形成许多中小尺度结构，导致耗散明显增强，相当于增加了一个高阻值的有效电阻–超电阻，对磁重联的性质和能量转换效率产生重要的影响。已有的研究表明，超电阻的出现是非线性过程的直接后果；另外，在电流片内的湍流在不同的位置上会同时呈现出各向异性和各向同性的特征。本项目将通过高分辨数值实验来刻画电流片内湍流的精细结构，分析不同性质的湍流结构的频谱特征和演化规律，了解这些特征和规律随磁雷诺数的变化方式，深入探讨大尺度环境中磁重联的特殊本质，对超电阻产生的条件与物理本质进行精细化研究。

In the universe, both the lengthscale and the electrical conductivity are usually very large, with the magnetic Reynolds number, Rm (or equivalently, the Lundquist number, S), varying from 10^6 to 10^12 in the solar corona, and from 10^15 to 10^21 in the interstellar medium, respectively. The magnetic field diffusion in such an environment is thus confined to very local regions including X-points, current sheets (CS), quasi-separatrix layers, null points, and magnetic separators...A long CS in the corona forms when the eruption severely stretches the closed coronal magnetic field as predicted by the Lin-Forbes model. It appears between the coronal mass ejection (CME) and the associated flare. Magnetic reconnection in a CS converts magnetic energy into heating (accounting for the flare) and kinetic energy of the bulk plasma (accounting for the CME), together with the production of energetic particles. Observations further indicated that that a CME/flare CS could as well possess a very large thickness, d, according to observations, more than 10^4 km thick, comparing the traditional theoretical expectation of a few 10^2 meters thick...Studying the information about the value of d, we notice that the apparent value of d deduced from observations generally ranges from a few times 10^3 km to a few times 10^5 km, no matter by what instrument, in which wavelength, via which approach, and in which event the observations were made. Although the observed values of d span three orders of magnitudes, the range of the difference is still small compared to that between the traditional expectation and the observed ones, which spans seven to eight orders of magnitudes. ..Numerical simulations indicated that the thickness of the CS may decrease continuously at the beginning of the reconnection process due to the pressure of the reconnection inflow, but the decrease in the thickness will cease after the turbulence commences inside the CS. It is now widely accepted that the turbulence results from the tearing mode instability occurring in the long CS, and the formation of turbulent structures produces an extra pressure to neutralize the impact of the reconnection inflow...This indicates that a very thin CS is not the necessity for reasonably fast reconnection required for the major solar eruption. Both increasing the resistivity and decreasing the size of the diffusion region can speed up reconnection, and decreasing the size contributes much more impact onto enhancing the diffusion that is dependent on the size in an inversely quadratic fashion. In the corona, the resistivity does not change too much, but the size can change dramatically either through decreasing the scale of the diffusion region or via producing small scale structures in the region. The tearing mode creates small structures, which allows the dissipation of a large amount of magnetic energy to occur in many small pieces simultaneously, enhancing the diffusion...Enhancing diffusion is equivalent to an extra resistivity in the reconnection region, which is also known as the “hyper-resistivity”. It is non-linear MHD property of the turbulent plasma, and results from the non-linear interaction between the perturbation to the velocity and that to the magnetic field, and occurs in the competition between the amplification and the diffusion of the magnetic field by the tearing mode turbulence. It is impossible for individual particles to govern the CS scale...Overall, the questions of the geometric scale and the fine structures inside the CS still remain open. We need continuously to perform numerical experiments to look into this question in detail, to identify the role of each agent mentioned above in contributing to the hyper-resistivity, as well as the detailed physical property of hyper-resistivity, to distinguish how these agents govern the dissipation, and to study carefully the implication and the importance of these agents to the particle acceleration by reconnection.