One year's worth of magnetospheric plasma analyzer data from three Los Alamos geosynchronous satellites are used for a statistical study of proton and electron fluxes at geosynchronous orbit and their dependence on local time (LT) and geomagnetic activity level as measured by Kp. When displayed as a function of LT and Kp, the fluxes exhibit distinct boundaries, which are shown to be consistent with a combination of a global pattern of particle drift through the magnetosphere and loss processes mainly due to charge exchange of the ions and auroral precipitation of the electrons. A Hamiltonian energy conservation approach combined with the (U, B, K) coordinate transformation introduced by Whipple [1978> is used to calculate the theoretical position of the separatrix between open and closed drift trajectories (Alfv¿n layer) as a function of particle species, energy, local time, and geomagnetic activity level. The comparison of the theoretical boundaries with the observations confirms the predictions of plasma sheet access to the geosynchronous region. The analysis also provides independent statistical support for previously derived relationships between Kp and the strength of the global convection electric field. ¿ 1999 American Geophysical Union