The Density of the Earth's Cusp Region: Three‐Dimensional Distribution, Formation, and Influencing Factors

Abstract The Earth's magnetospheric cusp region is an important pathway for solar wind particles entering the magnetosphere and ionospheric ions escaping. The plasma density is a key parameter for understanding this physical process. Based on measurements from the Cluster‐3 spacecraft (2001–2010), we selected 878 cusp crossing events (470 in the northern and 408 in the southern hemisphere) and statistically studied the three‐dimensional distribution of cusp ion density, its causes, and influencing factors. Our analysis revealed that cusp density is highest at MLT = 12 and decreases as MLT moves away from 12 in both hemispheres; in Solar Magnetic coordinates, the maximum density occurs near the equatorward boundary in the X direction, while in the Z direction, the density shows slight changes. This feature may be largely influenced by the inhomogeneity of magnetic pressure. Furthermore, we found that as the dominant source, solar wind ions enter the cusp effectively near noon and diffuses toward dawn and dusk; the polar ionosphere source becomes important with increasing AE index, transporting particles from a wide MLT range to the cusp; and the dipole tilt angle significantly affects cusp density.