Physical properties of electron variations of periods in the range 2--12 min detected in the afternoon magnetosphere at synchronous altitudes by the University of Minnesota electron spectrometer on the ATS-1 satellite are presented. We find that in the energy interval 50 keV to 1 MeV most of the electron variations are correlated with the intensity of the local magnetic field. This correlation exists down to a time scale of a few minutes. The pitch angle analysis show that the enhancement of fluxes occurs primarily at large pitch angles. These characteristics can be accounted for by adiabatic causes. When it is assumed that the perturbed magnetic field is time dependent but homogeneous in space along the dipole direction, an expression for the change of particle fluxes in the equatorial plane is derived in terms of measurable quantities. We show here that the derived expression agrees well with observed characteristics of trapped electron fluxes measured by the University of Minnesota electron spectrometer on ATS-1. In particular, the quasi-periodic modulation characteristics observed here are satisfactorily explained.