Measurements of geomagnetically trapped 55‐Mev protons made before and after the detonation of the Starfish nuclear device revealed that at low altitudes the flux increased considerably after the Starfish burst. Theoretical analyses presented in this paper indicate that the flux change resulted from a redistribution of the existing trapped protons in the inner zone. Two processes were found to be capable of altering the proton distribution appropriately: (1) interaction of the protons with hydromagnetic waves produced by the burst and (2) multiple reflections of the protons from a magnetic field inhomogeneity that propagates along magnetic field lines with the local hydromagnetic velocity. If the hydromagnetic velocity is determined from approximate ambient plasma densities, the latter mechanism gives an altitude dependence of the increased flux that is qualitatively in agreement with the measurements. However, much higher plasma densities, or repeated traversals of the field line by the magnetic inhomogeneity, are required to explain the magnitude of the observed flux chang