用Particle-in-Cell模拟无碰撞、应力、X点坍缩期间的磁重联
Magnetic reconnection during collisionless, stressed, X-point collapse using Particle-in-Cell simulation
摘要：
考虑了弱应力和强应力X点坍塌两种情况。这里，描述符弱和强分别指X点的20%和124%单向空间压缩。在弱应力情况下，重连率定义为X点（磁零点）的面外电场，由外部磁场和阿尔文速度的乘积归一化，峰值为0.11，其平均值超过1.25阿尔文倍为0.04。在光谱的高能量端，电流片中的电子能量分布示出指数随时间变化的幂律分布，在最终模拟时间步长（1.25阿尔文倍）处达到最大值-4.1。在强应力的情况下，磁重联峰值出现快3.4倍，并且更有效。峰值重连率现在达到2.5，平均重连率超过1.25阿尔文倍为0.5。电流片中电子的幂律能谱现在达到一个更陡的指数-5.5，这个值接近于在地球磁尾X型区域附近观察到的值。在大约一个阿尔文时间内，在弱应力和强应力的情况下，初始磁能的2%和20%分别转化为热能和加速粒子能。在这两种情况下，在重联的峰值期间，产生了四重面外磁场，暗示可能重联的霍尔机制。这些结果强烈地表明了无重粒子、受应力的X点坍缩作为解决太阳日冕加热问题的一个可能的贡献因素的重要性，或者更一般地说，作为将磁能转化为热能和超热粒子能的一种有效机制。
英文摘要：
Two cases of weakly and strongly stressed X-point collapse were considered. Here descriptors weakly and strongly refer to 20 % and 124 % unidirectional spatial compression of the X-point, respectively. In the weakly stressed case, the reconnection rate, defined as the out-of-plane electric field in the X-point (the magnetic null) normalised by the product of external magnetic field and Alfvén speeds, peaks at 0.11, with its average over 1.25 Alfvén times being 0.04. Electron energy distribution in the current sheet, at the high energy end of the spectrum, shows a power law distribution with the index varying in time, attaining a maximal value of -4.1 at the final simulation time step (1.25 Alfvén times). In the strongly stressed case, magnetic reconnection peak occurs 3.4 times faster and is more efficient. The peak reconnection rate now attains value 2.5, with the average reconnection rate over 1.25 Alfvén times being 0.5. The power law energy spectrum for the electrons in the current sheet attains now a steeper index of -5.5, a value close to the ones observed in the vicinity of X-type region in the Earth's magneto-tail. Within about one Alfvén time, 2% and 20% of the initial magnteic energy is converted into heat and accelerated particle energy in the case of weak and strong stress, respectively. In the both cases, during the peak of the reconnection, the quadruple out-of-plane magnetic field is generated, hinting possibly to the Hall regime of the reconnection. These results strongly suggest the importance of the collionless, stressed X-point collapse as a possible contributing factor to the solution of the solar coronal heating problem or more generally, as an efficient mechanism of converting magnetic energy into heat and super-thermal particle energy.
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