Does the F2-layer follow the constant pressure level?

A problem concerning a relation between the F2 layer maximum height and constant pressure level causing difficulties in explanation of negative ionospheric disturbances is discussed. It is shown that the F2 middle-latitudinal layer maximum does not relate to the constant pressure level during its variation, but in the daytime it is rigidly related to the specified O/N2 ratio level, which in its turn, is unambiguously related to O/N2 at the fixed height and to the atmospheric temperature. In this connection there is no ground for revising the recently accepted mechanism of negative disturbances in the F2 region