Particle-in-Cell Simulation on the Electron Dynamics at the Dayside Magnetopause in a Small Magnetosphere

By performing three-dimensional full particle-in-cell plasma simulations, we have been studying a small magnetosphere, which is formed by the interaction between a weakly magnetized small body and the solar wind. In this study, we mainly focus on the electron dynamics at the dayside magnetopause in the equatorial plane. In the simulation model, we place a weakly magnetized small body at the center, and we keep injecting the solar wind having the southward IMF from one of the boundaries of the simulation domain. We define D_p as the distance between the dipole center and a position where the dynamic pressure of the solar wind balances the dipole magnetic field pressure at the dayside. We focus on a small-scale magnetosphere with λ_i/D_p=1 where we define λ_i as the ion inertia length. In the simulation results, we found dawn-dusk asymmetry characteristics of the spatial plasma distribution in the equatorial plane. It is also interesting to observe intense electron flux along the dayside magnetopause in the equator. By the analysis of particle trajectories and the electron velocity distribution functions, we found that this flux is due to the electrons accelerated by the local electric field enhanced at the magnetopause. They encounter the boundary where the southward IMF and the northward magnetic dipole field cancel each other, and they start making meandering motions toward the dawn side. The intense electron flux is due to the meandering motion of electrons which are accelerated by the local electric field. We also found the density fluctuation at the region where electrons make meandering motions. To reveal the reason for the density fluctuation, we performed a one-dimensional PIC simulation. We found that the density fluctuation can be due to the current-driven instability, which is triggered by the accelerated electrons meandering along the magnetopause.