Roles of initial current carrier in the distribution of field-aligned current in 3-D Hall MHD simulations

Abstract

A three-dimensional (3-D) Hall MHD simulation is carried out to study the roles of initial current carrier in the topology of magnetic field, the generation and distribution of field aligned currents (FACs), and the appearance of Alfven waves. Considering the contribution of ions to the initial current, the topology of the obtained magnetic field turns to be more complex. In some cases, it is found that not only the traditional B y quadrupole structure but also a reversal B y quadrupole structure appears in the simulation box. This can explain the observational features near the diffusion region, which are inconsistent with the Hall MHD theory with the total initial current carried by electrons. Several other interesting features are also emerged. First, motions of electrons and ions are decoupled from each other in the small plasma region (Hall effect region) with a scale less than or comparable with the ion inertial length or ion skin depth d i =c/ω p . In the non-Hall effect region, the global magnetic structure is shifted in +y direction under the influence of ions with initial y directional motion. However, in the Hall effect region, magnetic field lines are bent in −y direction, mainly controlled by the motion of electrons, then B y is generated. Second, FACs emerge as a result of the appearance of B y . Compared with the prior Hall MHD simulation results, the generated FACs shift in +y direction, and hence the dawn-dusk symmetry is broken. Third, the Walen relation in our simulations is consistent with the Walen relation in Hall plasma, thus the presence of Alfven wave is confirmed.