Propagation of radio waves through the region of enhanced (cold) ionization of the ionosphere formed below the ionospheric E¿layer by the X¿ray and penetrating radiation of a space nuclear explosion is considered. A method to calculate the field and the main radiophysical effects caused by this region is developed. The method makes it possible to calculate the above indicated effects for a wide range of the equivalents and altitudes of the nuclear charge explosions. Asymptotic formulae are obtained for the attenuation determining the conditions of radio wave propagation through the lower region of the enhanced ionization. The formulae make it possible to estimate the effects with the error not exceeding the error in the initial data on spatial-time distributions of the electron concentration.