The cause of Pi 2 pulsations at substorm onset can be understood in terms of the transient ionospheric response to an enhanced convection in the plasma sheet. A two-dimensional model is presented to demonstrate how the transient ionospheric response can lead to damped oscillations similar to the Pi 2 pulsations observed at substorm onset. Alfv¿n waves are launched by the enhanced convection and subsequently reflected between the ionospheres in the opposite hemisphere. The ionospheric conductivity is fully anisotropic and highly nonuniform.