MAGNETOPAUSE ENCOUNTERS IN THE MAGNETOTAIL AT DISTANCES OF SIMILAR-TO-80 R(E)

The GEOTAIL satellite experienced several encounters with the magnetop ause during the interval 0300 to 0600 hours on 27 October 1992 at a gs m location of (X,Y,Z) similar to(-81,-8,7.5) R(e). Energetic ions of b oth solar wind and ionospheric origin (H+, He++, and O+) were observed streaming in the anti-solar direction on both the lobe and magnetoshe ath side of the plasma mantle-magnetosheath boundary, with maximum flu xes occurring at the boundary. Even though the mantle plasma had penet rated well into the lobe through convective drift, the energetic ions were observed only at the magnetopause and provided a clear signature of the boundary region. Using a measured convective drift velocity and observed energetic particle anisotropies, we estimated that the maxim um distance from GEOTAIL that the energetic ions could have been accel erated or made contact with the boundary field lines was < 8-39 R(e), implying that both solar wind and ionospheric ions are accelerated and contact the magnetopause many tens of earth radii behind the earth. W e further found that the unusual magnetopause positions encountered at this time could be explained in part by solar wind aberration effects . However additional processes (e. g., magnetospheric breathing modes, convoluted tail topologies) are required to fully explain these obser vations.