Basic properties of large scale irregularities in the rnid-latitude ionospheric F region and their relationship with atmospheric gravity waves are studied. The temporal evolution of the plasma Perkins instability is analyzed. It is found that the instability saturates at a small amplitude. The coupling between the Perkins instability and gravity waves is discussed and it is shown that gravity waves can initiate the plasma instability in the mid-latitude F region. The plasma instability initiated by gravity waves can evolve into steady-state perturbations, large amplitude wave-like structures, and spatially and temporally modulated irregularities. The theory indicates the production mechanism of the large scale irregularities in the mid-latitude ionosphere.