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It is shown that the flux increases of trapped ions and electrons observed by Explorer 45 at L≤4 during two large geomagnetic storms can be quantitatively explained by a 1–3 RE inward radial displacement of the preexisting trapped particle distribution. This proposed source for the storm time ring current at L≤4 requires only the acceleration of the previously existing trapped particle population via inward displacement under conservation of the first two adiabatic invariants. No source of new trapped particles at L≤4 is required. It was not possible to test whether such an inward radial displacement can account for storm time flux increases at any radial distances beyond L=4; however, the existence of particle losses implies that new particles are injected into the trapping region beyond L=4. The required inward radial displacements of the trapped particles can be explained by an equatorial electric field with an average azimuthal component of ∼0.3–1.0 mV/m, which is a factor of 2 or 3 greater than electric fields observed within the plasmasphere in association with typical, nonstorm time, substorm activity. It is suggested that a significant difference between large geomagnetic storms and typical substorm activity may be inward convection occurring over a large (≳270°) longitude range during storms but only over a small (≲90°) longitude range during typical substorms.