Characteristics of the exterior cusp for steady southward interplanetary magnetic field: Interball observations

High-latitude observations of plasma flow across the magnetopause are made by the Interball-Tail satellite near the polar cusp for steady southward an d dawnward interplanetary magnetic field. Three-dimensional proton distribu tion functions measured in this region contain the features predicted by th e existing reconnection models. D-shaped distributions in magnetospheric pa rts of the field lines with cutoff near zero velocity parallel to the magne tic field and tablet-shaped distributions tailward from the cusp are observ ed and can be interpreted in terms of a gradually diminishing ion injection . Besides these predicted signatures, a new type of proton distribution wit h large pitch angles is also observed. We present observations of suprather mal ions and electrons with peaks at similar to 90 degrees pitch angles on the high-latitude magnetospheric field lines adjacent to the magnetopause. These particles show a sharp change in fluxes on the magnetospheric side, i ndicating that they are probably quasi-trapped in the cusp field. We also p resent the results of a simulation that uses the Toffoletto-Hill model of t he open magnetosphere to trace the particle transport from the magnetosheat h to magnetospheric locations of Interball. The simulation successfully rep roduces the main predicted features of distribution functions as well as di stributions peaked at large pitch angles. The results show that for steady southward interplanetary magnetic field with a large dawnward component, th e entry of magnetosheath plasma into the magnetosphere can be described in terms of open topology of field lines near the high-altitude cusp.