|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.