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Among all earthquake precursors, those related to electromagnetic effects are the most puzzling, and the many possible sources of noise are cause of lively controversies. A large number of laboratory experiments clearly suggest that micro-fracturing is associated with the appearance of spontaneous charge production (electrification) and transient Electric or ElectroMagnetic (EM) Emission. Many electric and magnetic pre-seismic and co-seismic effects have been reported in the past as well as ionospheric perturbations. The aim of this paper is to review some of these ionospheric perturbations performed in the former Soviet Union. The importance of sporadic E-layer formation is underlined. Statistical studies with the available observations are done. At the end, a physical model of lithosphere-ionosphere coupling is presented in order to explain the observations. This model takes into account the following effects at the Earth's surface: electric charge generation, emanation of radioactive gas, temperature variation, and surface vibration. It is shown that these effects can trigger sporadic E-layer formation.