A normalized statistical study of Earth’s cusp region based on nine-years of Cluster measurements

Nine years (2001–2009) of data from the Cluster spacecraft are analyzed in this study of the Earth’s mid- and high-altitude (2–9R_E) cusp. Properties of the cusp region, and its location and size in the Solar Magnetic coordinate system, are studied statistically. The survey shows that (1) the relationships between X and Z are nearly linear for the poleward, equatorward boundaries and the center of the cusp; (2) the relationship between cusp width in the X direction and Z can be expressed by a quadratic function; (3) the cusp region is almost dawn-dusk symmetric for the cusp width in the X direction. Based on topology information, a new normalized statistical methodology is developed to organize the measurements of cusp crossings to obtain distributions of magnetic field and plasma parameters in the XZ plane. The statistical results show that (1) B_x is mostly negative and B_z is always negative; (2) proton velocity is found to be positive for V_x and V_z at low altitudes, while V_x and V_z are negative on the equator side and negative V_x and positive V_z on the pole side at high altitudes; (3) proton density is higher on the equator side than on the pole side. Results reported here will be useful in suggesting directions for future cusp research.