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Electron density irregularities at medium scale (from a few kilometers to tens of kilometers) have been observed in the high-latitude topside F region by the satellite Aureol 3. These irregularities are field-aligned structures, as evidenced by their ducting effect on downgoing ELF hiss. It is often observed that the density enhancements have an asymmetrical structure in the direction perpendicular to the magnetic field. On one side of the density enhancement, an intense electrostatic turbulence is observed together with large density fluctuations. The other side is usually steeper and appears stable. These observations suggest that the density irregularities are convected by the dc electric field and that the instabilities are of the gradient drift type.