High-altitude cusp: INTERBALL observation Original Research Article

The cusp is a funnel-shaped part of the Earthʹs magnetosphere where the magnetospheric magnetic field lines are directly interconnected with the magnetosheath ones. The magnetic field configuration allows the magnetosheath plasma to precipitate toward the ionosphere. This feature is used for the determination of the cusp position in low altitudes. However, it is often difficult to distinguish different plasma populations in high altitudes. The cusp region is bounded with the low-latitude boundary layer (LLBL), entry layer or cleft on the equatorward side, and by the plasma mantle on the poleward side, but the plasma and magnetic field parameters are similar in all these regions. We have used the INTERBALL-1 and MAGION-4 satellites to study the topology and dynamics of high-altitude cusp regions under quiet solar wind conditions but different directions of the interplanetary magnetic field (IMF). Two-point event studies have shown that (1) the topology of the magnetic field in the high-altitude cusp is controlled by the IMF direction, (2) the cusp plasma source is located near the tailward boundary of the cusp during northward IMF, and (3) magnetosheath fluctuations are correlated with the fluctuations of the cusp precipitation.