The density and bulk velocity distributions of warm magnetosheath particles and cold ionospheric O+ and H+ ions are calculated along a polar cusp (cleft) magnetic field line. The warm collisionless plasma is injected at 12 earth radii, and interacts with the cold polar wind electrons and ions of ionospheric origin. After magnetic reflections in the exosphere (above 1400km altitude) most of the ions move upwards along tail magnetic field lines and feed the plasma mantle flow. The bulk velocity of these warm protons is approximately 200km s-2 and happens to be proportional to the average thermal speed of the magnetosheath ions at their injection point in the magnetosphere. The density in the plasma mantle is found to be about half that in the entry layer. The electric potential distribution is deduced from the quasi-neutrality condition, A large parallel electric field corresponding to an electrostatic double layer is obtained for the assumed boundary conditions.