The contribution of ionospheric plasma to the Earth's magnetosphere has been recognized for more than 3 decades. The magnitude of this contribution has become more evident over that same time period with the observed magnitude of the low‐energy ionospheric supply increasing as the measurement techniques improved. Estimates based on Dynamics Explorer measurements in the mid‐1980s suggested that the ionospheric plasma supply is sufficient to populate the plasmasphere, plasma trough, plasma sheet, and magnetotail lobes. Recent measurements from the Thermal Ion Dynamics Experiment on the Polar spacecraft have been used in conjunction with an ion trajectory model to reexamine the process and magnitude of the ionospheric supply of magnetospheric plasma. These measurements reveal the energy, pitch angle, and flux characteristics of the upward flowing polar wind over broad regions of the high‐latitude ionosphere. Onboard measurement of spacecraft potential is found to be a fundamental element in interpreting the measured ion outflow. Newly derived polar wind fluxes are determined to be near 6.0 × 107 ions cm−2 s−1 at 5000 km altitude during local winter and magnetically quiet conditions. Using the measured ionospheric source characteristics in combination with the trajectory code reveals the nature of the ionospheric/magnetosphere filling process and shows that the ionospheric source is sufficient to supply the observed densities and energies of the plasma sheet and magnetotail lobes. Many of the ionospheric particles are further transformed to ring current energies and locations after circulation through the plasma sheet. This measurement/calculation approach is able to show which regions of the high‐latitude ionosphere are important for plasma sheet/ring current filling. The ionospheric sources used in the calculations include the dominant polar wind, the cleft ion fountain, and the auroral zone.