Bursty bulk flows (BBFs) are known to exhibit abrupt increases in plasma flow velocity and in the intensity of the northward/southward magnetic fields, occasionally followed by periodic variations in a characteristic time of 1--3 min. In this paper a theoretical model for BBFs is proposed. In this model a region of a dawnward electric field is assumed to propagate earthward, corresponding to the propagation of the variation front associated with magnetic reconnection. The coupling of the magnetosphere and the ionosphere is modeled using an equivalent electric circuit. We conducted a series of test particle simulations with the assumption that each test particle corresponds to a magnetic flux tube. We also obtained an analytic solution that approximates the numerical results and explains the main characteristics of BBFs.