Ion Beams in the Plasma Sheet Boundary Layer: MMS Observations and Test Particle Simulations

Abstract

AbstractUsing Magnetospheric Multiscale (MMS) observations and combined MHD/test particle simulations, we further explore characteristic ion velocity distributions in the plasma sheet boundary layer. The observations are characterized by earthward beams, which at a slightly later time are accompanied by weaker but faster tailward beams. Two events are presented showing different histories. The first event happens at entry from the lobe into the plasma sheet. Energy‐time dispersion indicates a source region about 25 tailward of the satellite. The second event follows the passage of a dipolarization front closer to Earth. In contrast to earlier MHD simulations, but in better qualitative agreement with the first observation, reconnection in the present simulation was initiated near . Simulated distributions right at the boundary are characterized by a single crescent‐shaped earthward beam, as discussed earlier (Birn, Hesse, et al., 2015, https://doi.org/10.1002/2015JA021573). Farther inside, or at a later time, the distributions now also show a simple reflected beam, evolving toward a more ring‐like distribution. The simulations provide insight into the acceleration sites: The innermost edges of the direct and reflected beams consist of ions accelerated in the vicinity of the reconnection site. This supports the validity of estimating the acceleration location based on a time‐of‐flight analysis (after Onsager et al., 1990, https://doi.org/10.1029/GL017i011p01837). However, this assumption becomes invalid at later times when the acceleration becomes dominated by the earthward propagating dipolarization electric field, such that earthward and tailward reflected beams are no longer accelerated at the same location and the same time.