Schematic geometric arrangement for the toy example of cylindrical depletion region embedded in the slab of the ambient ionospheric medium (a). Three typical scenarios of signal propagation through the considered ionospheric structure are marked with i, ii, iii. Namely, in case i, the refractive scattering of the signal occurs at the top and bottom sides of the depleted structure; in case ii, the additional scattering occurs at the vertical wall of this structure, while in case iii, no such scattering occurs since the depleted region is absent. An example of the vector fields used to construct the phase gradient screen is shown in (b) and (c). The screen itself is placed perpendicular to the initial propagation direction of the signal wave and has a circular cross-section with the cylindrical depletion region. Figure (b) shows the contribution to the displacement vector field δr₁ that comes from the scattering on the lateral surface of the cylinder, cf. the ray path ii in scheme (a). Figure (c) shows the generated phase screen with random amplitude δφ and the corresponding gradient field ∇_⊥δφ shown as black arrows.