The attachment of free electrons to neutral molecules followed by detachment from the resulting negative ions leads to fluctuations of the electron density in the lower ionosphere. Such fluctuations, superimposed on thermal fluctuations, enhance the scattering cross-section for an incident electromagnetic wave. A simple analytical expression of the cross-section is demonstrated independently by a thermodynamical method and by a stochastic formulation. Furthermore, a numerical simulation of the fluctuating electron concentration leads to a numerical cross-section profile which agrees with the theoretical expression.