3-D Global Hybrid Simulations of Mode Conversion at the Dayside Magnetopause

Abstract

Mode conversion at the magnetopause has been suggested to lead to the generation of kinetic Alfven waves (KAWs) and effective mass transport from the solar wind into the magnetosphere. To investigate the mode conversion process in the dynamics dayside system, a systematic 3-D global hybrid simulation that includes the solar wind, bow shock and the geomagnetic field is carried out for the dayside magnetosphere under various interplanetary magnetic field (IMF) conditions. In the quasi-parallel (Q-||) shock, the foreshock compressional pulses are self-consistently generated by the interaction between the solar wind and the dipolar geomagnetic field. The simulation results show that as the compressional pulses propagate from the magnetosheath to the magnetopause, short wavelength structures of k⊥ ρi ~ 1 with enhanced parallel electric field E|| are excited in the magnetopause boundary layer (MPBL), where k⊥ ~ kx is the perpendicular wave number nearly along the GSE x direction, and ρi is the ion Larmor radius. The wave phase relationship between the magnetic field and the density changes from in-phase in the magnetosheath to anti-phase in the short-wavelength MPBL perturbations. The wave polarization is predominantly compressive in the magnetosheath, whereas strong transverse wave powers appear abruptly around the MPBL. The mode conversion from the compressional pulses to KAWs is identified around the predicted Alfven resonance points in the MPBL. As these KAWs evolve, KAW modes dominated by azimuthal wave number with ky ρi ~ 1 are also generated. The short-wavelength KAWs in the MPBL are identified by the sharp increases in E|| and the transverse electromagnetic field polarization relation of Alfven mode. These KAW perturbations propagate poleward into the cusps along the MPBL. Due to the differential flow convection speeds at various latitudes, the KAW packets expand along the north-south direction, while they may also merge with newly formed KAWs generated by newly-arrived compressional waves.

The mode conversion takes place downstream of the Q-|| shock, where the driver compressional waves are the strongest in their global distribution. In the cases with a radial IMF, the mode conversion takes place in the subsolar region. In the cases with a more general IMF direction, the Q-|| shock regions and thus the strongest mode conversion shift off the subsolar, although the KAWs may still be identified near the subsolar region around the MPBL. In the dipole field region of the magnetosphere, field line resonance associated with the standing shear Alfven waves is found due to the mode conversion. In the presence of the IMF By component, the field line oscillations in the inner magnetosphere are more intense downstream of the Q-|| shock than on the quasi-perpendicular (Q-⊥) side, indicating again that the KAWs from the mode conversion is in favor of the Q-|| side. Over several periods of incoming magnetic pulses, the KAWs approximately show a symmetric pattern from north to south. This symmetry, however, is not observed in the existence of the northward component (Bz in GSM) in the IMF, because the Q-|| is now located in the southern hemisphere where more KAWs could be perceived. While observational signatures are provided by the global simulation, the simulation results are also compared with satellite observations for the physical evidence of mode conversion.