When th quasi-dc electric field exceeds about 25 mV/m in the ionosphere, a thermal instability that is caused by the Ohmic dissipation of Pedersen current in the electron gas can produce ionospheric irregularities in the F region within a few minutes. The threshold fields and scale lengths of the instability are determined by the electron energy loss due to electron-ion collisions, through which, as an intermediate step, energy is transferred from electrons to neutrals as energy sinks. The excited ionospheric irregularities have two scale length regimes: one is from tens of meters up to hundreds of meters, and the other one is from hundreds of meters up to a few tens of kilometers. This thermal instability is effective in generating high-altitude ionospheric irregularities, especially in the polar cap, where large quasi-dc electric fields have been observed even during quiet ionospheric conditions.