Three-Dimensional Hybrid Simulation Of Dayside Magnetic Reconnection

Abstract

With a three dimensional (3-D) global hybrid simulation model, we investigate magnetopause reconnection and the energy spectra of cusp precipitating ions under different IMF conditions.

First, the magnetic configuration and evolution of Flux Transfer Events (FTE) and the associated ion density and ion velocity distribution at various locations on the magnetopause are investigated under a purely southward IMF. The results reveal: (1) Multiple X lines are formed during the magnetopause reconnection, which lead to both FTEs and quasi-steady type reconnection under a steady solar wind condition. The resulting bipolar signature of local normal magnetic field of FTEs is consistent with satellite observations. (2) A plasma temperature rise is seen at the center of an FTE, compared to that of the upstream plasma in the magnetosheath. The temperature enhancement is mainly in the direction parallel to the magnetic field due to the mixing of ion beams. (3) Flux ropes that lead to FTEs form between X lines of finite lengths and evolve relatively independently. The ion density is enhanced within FTE flux ropes due to the trapped particles, leading to a filamentary global density. (4) Different from the previous understanding based on the asymmetric density across the magnetopause, a quadrupole magnetic field signature associated with the Hall effects is found to be present around FTEs. (5) A combination of patchy reconnection and multiple X-line reconnection leads to the formation of reconnected field lines from the magnetosphere to IMF, as well as the closed field lines from the magnetosphere to the magnetosphere in the magnetopause boundary layer.

Secondly, both the spatial and temporal energy spectra of cusp precipitating ions are computed by tracing trajectories of the transmitted magnetosheath ions under a southward IMF. The spatial spectrum shows a dispersive feature consistent with satellite observations, with higher energy particles at lower latitudes and lower energy particles at higher latitudes. The simulation reveals (1) how and where particles are transmitted from the solar wind into the magnetosphere via direct magnetic reconnection on the dayside; (2) how the features of the spectra are related to ongoing magnetic FTEs; (3) how the motion of the cusp, particularly the latitudinal variation of the open/closed field line boundary, is correlated with the dayside reconnection and reflected in the spectra, energy flux due to precipitating ions as a function of time.

Third, the energy spectra of cusp precipitating ions and magnetopause reconnection under an IMF of a finite $B_y$ component are investigated. It is found that component reconnection is the reconnection process at the dayside magnetopause. Dispersive feature is also shown in spatial spectra for precipitating ions in the cusp, compared to that under a purely southward IMF. When IMF clock angle is larger than 180 degree, the heaviest precipitation shifts to the dawn side.