Latitude Variations of Cusp-Related Boundaries Dependent on Solar Wind Conditions and Dipole Tilt: MHD Simulations and Auroral Observations

The magnetospheric cusps are populated by the magnetic field lines that connect upward to the magnetosheath and extend downward to the ionosphere, therefore the magnetosheath plasma has direct access to the polar ionosphere in these regions. The location of the cusp responds dynamically to solar wind conditions and geomagnetic field, influencing magnetosphere-ionosphere coupling. The equatorward boundary of the cusp is adjacent to the low-latitude boundary layer (LLBL)/cleft, where the dayside open-closed boundary (OCB) is typically located. The polar cap boundary (PCB) delineates the extent of open magnetic flux, and its midday position is associated with the cusp and OCB. Particles precipitating in the cusp contribute to midday auroral emissions and field-aligned currents. The latitude of midday auroral equatorward boundary varies with the cusp's equatorward boundary, OCB, and the thickness of the LLBL. Field-aligned currents connect magnetospheric currents with ionospheric currents, with the Region 1 currents observed on both open and closed field lines. Consequently, the Region 1 current's high-latitude boundary near local noon relate to the cusp’s equatorward boundary dynamics. Despite the known associations between these cusp-related boundaries, their dynamic responses to variations in solar wind parameters and dipole tilt have not been fully characterized. This study investigates the latitude variations of midday auroral equatorward boundary, OCB footprint in the ionosphere, PCB, Region 1 current poleward boundary, utilizing DMSP auroral observations and CCMC MHD simulation results. The analysis reveals that:

• All boundaries shift equatorward with increasing southward IMF B_z, consistent with enhanced dayside reconnection.
• The boundaries exhibit systematic responses to IMF B_y and solar wind velocity, reflecting asymmetric convection and magnetospheric compression.
• All boundaries in the Northern Hemisphere shift with dipole tilt.
• The latitude of the midday auroral lowest-latitude boundary shows seasonal variations and solar cycle dependence.

These findings provide insights into the dependence of cusp location on solar wind conditions and dipole tilt, as well as the dynamic relationships between cusp-related boundaries, emphasizing the cusp’s role in solar wind-magnetosphere-ionosphere coupling.