A transient response model of Pi 2 pulsations due to an enhanced convection in the plasma sheet is presented. A nonlinear Alfv¿n wave is launched by the enhanced convection electric field. The reflection coefficient at the ionsphere is derived by matching the field-aligned current of the Alfv¿n wave to the field-aligned current from the divergence of the ionospheric Hall and Pederson currents. The resulting transient magnetic perturbation on the ground exhibits the characteristics of the Pi 2 pulsation signature accompanying a substorm onset. The predicted fundamental period of the magnetic perturbation is 4 times the one-way Alfv¿n travel time which increases toward higher latitude. The predicted Pi 2 source regions in the ionosphere are in reasonable agreement with observations.