Incoherent scatter data obtained at Saint-Santin have been analyzed between 1969 and 1972 in the height interval 120-200 km. The summer daytime electron density profiles show a maximum around 150 km followed by a minimum around 165 km, when the solar activity is low. The seasonal and solar cycle variations of this phenomena are discussed. When the day-time valley exists, the maximum density is ca. 20-30 per cent greater than the minimum value. The phenomenon cannot be fully explained on the basis of photochemical equilibrium. A wind-shear mechanism could play an important role.