Magnetic reconnection during collisionless, stressed, X-point collapse using particle-in-cell simulation

Magnetic reconnection during collisionless, stressed, X-point collapse was studied using a kinetic, 2.5D, fully electromagnetic, relativistic particle-in-cell numerical code. Two cases of weakly and strongly stressed X-point collapse were considered. Here the 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) normalized 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. During the peak of the reconnection, electron inflow into the current sheet is mostly concentrated along the separatrices until they deflect from the current sheet on the scale of electron skin depth, with the electron outflow speeds being of the order of the external Alfvén speed. Ion inflow starts to deflect from the current sheet on the ion skin depth scale with the outflow speeds about four times smaller than that of electrons. 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, the magnetic reconnection peak occurs 3.8 times faster and is more efficient. The peak reconnection rate now attains the value 2.5, with the average reconnection rate over 1.25 Alfvén times being 0.5. Plasma inflow into the current sheet is perpendicular to it, with the electron outflow seeds reaching 1.4 Alfvén external Mach number and ions again being about four times slower than electrons. The power-law energy spectrum for the electrons in the current sheet attains now a steeper index of $−5.5$⁠, a value close to those observed near the X-type region in the Earth’s magnetotail. Within about one Alfvén time, 2% and 20% of the initial magnetic energy is converted into heat and accelerated particle energy in the case of weak and strong stress, respectively. In both cases, during the peak of the reconnection, the quadruple out-of-plane magnetic field is generated, hinting possibly at the Hall regime of the reconnection. These results strongly suggest the importance of the collisionless, stressed X-point collapse as an efficient mechanism of converting magnetic energy into heat and superthermal particle energy.