We identify magnetospheric ions from Hydra on Polar and comprehensive plasma instrument on Geotail that have traveled between these spacecraft at three times at half-hour intervals from 1800 to 1900 UT on August 19, 1996. During this time the activity level was low, the magnetosphere was quiet, and inductive electric fields should have been minimal. The identification is made by first using the magnetic fields measured by Magnetic Field Experiment (Polar) and magnetic field instrument (Geotail), together with the Tsyganenko 1996 magnetic field model to calculate the mapping of pitch angles between spacecraft. We then make a one-parameter search in energy, and equivalently in the space defined by the first two adiabatic invariants, to find matchings in the ion velocity distribution functions as predicted by Liouville's theorem. We find matchings at these three times, with each matching occurring over the appropriate ranges of pitch angles and at a unique difference in kinetic energy. We interpret the difference in kinetic energy as a measure of the potential difference between the regions of the two spacecraft. These differences are consistent with the potential changes along each spacecraft path during this time from the electric fields measured by electric field instrument (Polar) and electric field detector (Geotail) and from a map of estimated magnetospheric potential distribution for the existing activity levels. Spacecraft positions when plotted in the (U, B, K) coordinate system confirm that ion travel between the spacecraft was possible during this time period. This technique for identifying particles traveling between spacecraft should be a valuable tool for correlating spectral features seen in particle data by different spacecraft in the magnetosphere. ¿ 1998 American Geophysical Union