Statistics of the Density Distribution in the Earth’s Cusp Region: Cluster Observations

The Earth's magnetospheric cusp serves as a key channel for solar wind particles to enter the magnetosphere and for ionospheric ions to escape. The plasma density is central to understanding of these processes. Based on Cluster observation (collected between 2001 and 2010) of 878 cusp-crossing events, we performed a statistical study on the three-dimensional distribution of cusp ion density, as well as the primary influencing factors.
Our findings show that the highest density occurs around MLT=12 in both hemispheres, declining as MLT shifts away from noon. In SM coordinates, the maximum density appears near the equatorward boundary along the X direction, while variations in the Z direction are relatively small. This feature can be explained by the inhomogeneity of magnetic pressure. We also observed that solar wind ions, as the main source, most effectively enter the cusp around noon and diffuse toward dawn and dusk. As the AE index grows, the polar ionosphere becomes increasingly important, supplying particles from a wide range of MLT values to the cusp. Finally, the cusp density is found to be correlated with dipole angle, suggesting the cusp’s ability to funnel charged particles depends on how Earth’s magnetic field is oriented.