Simulation of substorm‐time acceleration of oxygen ions on azimuthally directed magnetic field lines in the near‐Earth plasma sheet

Abstract

AbstractAcceleration of energetic ions is one of the drastic and common phenomena in the near‐Earth plasma sheet during the expansion phase of substorms. When a magnetic field line is stretched, the curvature radius of the magnetic field line becomes small, and many of energetic ions are accelerated nonadiabatically under the presence of electric fields. In a global magnetohydrodynamics (MHD) simulation, when the magnetic field line is further stretched, a flux rope structure appears in the near‐Earth plasma sheet (at ~8.5 RE for this particular simulation) for about 10 min before a substorm onset under the presence of the Y component of the interplanetary magnetic field. The magnetic field lines are azimuthally (in the east‐west direction) elongated near the equatorial plane, and the structure is different from that directly associated with magnetic reconnection. Based on a test particle simulation, we show that the oxygen ions departing in the flux rope structure a few minutes before the onset go around in the near‐Earth plasma sheet twice, experience strong dawn‐dusk electric field, and the ions gain kinetic energy as high as ~200 keV in ~10 min. The large acceleration results from nonadiabation motion together with geometry of magnetic field lines having a kink. The acceleration process (passing through or near the kink and energization by dawn‐dusk electric field) is not common. However, most of the particles that are accelerated more than 150 keV passed through or near the kink. The azimuthally elongated magnetic field line seems to have a large influence of substorm‐time acceleration of the oxygen ions that preexist before the onset.