The position of the trapping boundary for ?24-keV ions in the vicinity of the magnetopause has been determined using energetic particle three-dimensional distributions observed by ISEE 1 and 2. These observations have been compared with simultaneous plasma magnetic field measurements for the 0130--0135 UT interval on November 20, 1977. The results show that for this interval the magneto-sheath plasma penetration depth into the magnetosphere can be limited to a distance equal to the local plasma ion gryoradius of ≲100 km inside the position of the trapping boundary although penetration greater than 200 km was also observed. During this time the trapping boundary was in continual, rapid motion with velocities of up to ~90 km/s. This motion could complicate the geometry and therefore explain the apparent deeper penetration of the magnetosheath plasma a few ion gyroradii inside the trapping region. If the trapping boundary, determined by this energetic particle sounding technique, is the last closed field line, then the observation of magnetosheath plasma a number of ion gyroradii earthward of the trapping boundary indicates that magnetosheath plasma is able to penetrate onto closed fields lines.