Plasma characteristics of high-altitude cusp for steady southward-dawnward IMF Original Research Article

High-altitude observations of plasma flow across the magnetopause are made by the Interball-Tail satellite near the polar cusp for steady Bz < 0, By < 0 of the interplanetary magnetic field. Three-dimensional proton distribution functions measured in this region contain the features predicted by existing reconnection models. The evolution of the distribution function in magnetospheric parts of field lines tailward from the cusp from D-shaped distribution in the vicinity of the magnetopause to the pancake and torus distributions deeper in the magnetosphere can be interpreted in terms of a gradually diminishing ion injection. A simulation that uses the Toffoletto-Hill model of the open magnetosphere to trace the particle transport from the magnetosheath to magnetospheric locations of Interball is reasonably consistent with the observations. The results show that for the steady southward interplanetary magnetic field with a large By component the entry of magnetosheath plasma into the magnetosphere can be described in terms of open topology of field lines near the high-altitude cusp.