Dynamic processes and kinetic structure of collisionless reconnection at the dayside magnetopause: comparison between GEOTAIL observations and computer simulations

In this paper we report new kinetic features of ions and electrons observed in the vicinity of the reconnection layer on 10 January 1997. This event has a three-dimensional magnetic field topological structure, which is much more complex than the previously suggested two-dimensional magnetic configuration. The ion distributions are non-gyrotropic and electrons show non-Maxwellian distribution functions. Acceleration of multiple ion beams, both parallel and perpendicular to the local magnetic field, have been observed. The perpendicular acceleration of the multiple ion beams can be explained by plasma mixing between the meandering ions accelerated around the ion diffusion region and the cold ions convected directly from the magnetosheath without passing through the X-line region. The parallel acceleration of the multiple ion beams can be understood by the fact that high-velocity ions ejected from the vicinity of the X-line mix with the plasma flowing directly across the boundary. We observed the kinetic effect of the separation of the electron and ion edges due to the time-of-flight effect. It is stressed that kinetic processes are the key to understanding these new observations that cannot be adequately explained by magnetohydrodynamic (MHD) models.Key words. Space plasma physics (magnetic reconnection; charged particle motion and acceleration) – Magnetospheric physics (magnetopause, cusp, and boundary layers