Spreading of electron scale magnetic reconnection with a wave number dependent speed due to the propagation of dispersive waves

摘要

In this work, we study electron scale spreading of localized magnetic reconnection in the presence of a guide magnetic field, however, without the influence of ions and cross-scale coupling. These fundamental physics studies will help to understand the coupling of the electron scale spreading with the ion scales in real systems. An electron-magnetohydrodynamic (EMHD) model is employed to model the physics at electron scales. Three dimensional EMHD simulations and linear eigen mode analysis are performed for different guide field strengths. The simulations show a wave-like bi-directional spreading of magnetic reconnection at electron scales. The electron scale spreading, however, unlike the ion scale spreading by Alfven waves, is caused by the uni- and bidirectional propagation of the dispersive flow induced and whistler wave modes, respectively. The dispersive nature of the two wave modes makes the spreading-speed dependent on the wave numbers of the unstable tearing mode, which depend on the thickness of the electron current sheet and the strength of the guide field. A model of the speed of spreading is developed, in which the spreading-speeds parallel and anti-parallel to the guide field are given by linear combinations of the group speeds of the two wave modes. The model prediction of the spreading speeds agrees well with the speeds obtained from the simulation results. For small guide fields, the spreading is asymmetric being faster in the direction of the electron flow. On increasing the guide field, the spreading becomes increasingly symmetric, with the speeds of the order of electron Alfven speed in the guide magnetic field, due to the dominance of the whistler group speed in determining the speed of the spreading. As a consequence of the asymmetric spreading, the chain of alternate X-and O-points, formed due the growth of oblique tearing modes, extends farther in the direction of electron flow as compared to that in the direction of the guide magnetic field. Published by AIP Publishing.