It is shown that Alfv¿n waves can be generated in the magnetosphere by the deceleration of magnetospheric convection due to the dynamo process in the magnetosphere-ionosphere coupling. The growth rate of Alfv¿n wave packets bouncing between hemispheres is given by &ggr;o?(2/&mgr;0 &Sgr;l) [&agr;&mgr;02&Sgr;p2V0y2-1> where &agr; is a dimensionless constant (~0.5), &mgr;0 is the free space permeability, &Sgr;p is the height-integrated Pedersen conductivity, l is the field line length between hemispheres, V0y is the convection speed in the magnetospheric dynamo region. Alfv¿n waves become unstable under disturbed conditions when both &Sgr;p and V0y are enhanced. The resulting unstable Alfv¿n waves can contribute to the Pi pulsations observed during substorms.