A dynamic semikinetic simulation is used to study the effect of substorm dipolarization events on low-energy thermal plasmas initially distributed isotropically along closed tailward stretched magnetic field lines in the middle magnetosphere. The temporal variation of the magnetic field during dipolarization induces a perpendicular electric field that leads to E¿B drift and produces centrifugal and betatron ion acceleration. We find that the dipolarization process produces field-aligned ion streams split into components with higher and lower speeds, as generated by parallel forces identified here as the ''centrifugal force'' and the ''Coriolis force.'' The higher-speed component leads to ion precipitation which is highly dispersed in energy during the dipolarization event. The postdipolarization evolution of the lower-speed component produces energy-time spectrogram features similar to the ''ion bounce clusters'' observed at geosynchronous orbit and noted in the ''convection surge'' simulations of Mauk [1986>. An entirely new feature not found in previous dipolarization simulations is the formation of an equatorially trapped population whose flux is strongly peaked at the 90¿ pitch angle and whose overall level sharply declines at latitudinal boundaries within 5¿ of the equator. These equatorially trapped ion distributions are generated through the combined influence of equatorial focusing by the parallel centrifugal force and betatron perpendicular energization. The postdipolarization latitudinal density distributions contain a local maximum, due to the trapped ion population, at the equator, where the electric potential similarly contains a small peak. Hence the substorm dipolarization process provides a new, heretofore unrecognized mechanism for producing the characteristic equatorially trapped warm ion populations observed, for example, by Horwitz and Chappell [1979> and Olsen et al. [1987>. ¿ American Geophysical Union 1996