In plasmas rapid thermal energization can result when sharp impulses occur within ambient fields. A familiar example of impulsive fields occurs in the ion pick-up mechanism when newly-ionized particles in the presence of ambient electric and magnetic fields suddenly gain substantial gyrational energy. Motions of ions in the presence of a magnetic field can simlarly be disrupted by sudden impulsive changes in local electric fields over timescales shorter than the local ion cyclotron period. Impulsive external forces violate adiabatic invariants of motion, abruptly altering the distribution of kinetic energy and providing a mechanism for the transfer of energy from E¿B drift into gyrational motion. Orbit integration shows order of magnitude increases in the particle thermal energy when the initial gyration speed is small compared to impulsive changes in the drift speed. On average, small random impulses increase the gyration speed, energizing and broadening ion velocity-space distributions. This process will be shown to have an important influence on the outflux of warm ions from the magnetosphere. ¿ American Geophysical Union 1990