The nighttime midlatitude ionosphere is numerically simulated to study the evolutionary process of the irregularities that generate the electric field fluctuations observed by satellites. Ionospheres in both hemispheres are included in the simulation, and the evolution of irregularities is studied with given neutral winds in these regions. In each ionosphere, an equation of the field line integrated Pedersen conductivity is derived by integration of the ion momentum equation along the geomagnetic field line. Given initial perturbations are reformed to a sheet-like structure at the first stage of the simulation. After this reformation, the irregularities start to grow through plasma instability. The sheet-like structures are also formed in the conjugate ionosphere, where no initial irregularities are given, when the irregularities are sufficiently developed. In a case of an intense neutral wind applied only in the source hemisphere, the growth rate is comparable to that predicted by the linear analysis which includes the electrical coupling between the conjugate ionospheres. When the intense neutral wind is applied in both the conjugate ionospheres, the growth of irregularities is affected by the electric currents in both hemispheres even when the conjugate ionosphere is in winter. The electric current of the conjugate ionosphere can grow or damp the irregularities depending on its direction. ¿ 1998 American Geophysical Union