Modelled dynamic spectra showing the relative signal intensity as a function of time and frequency for comparison with the observation of Cygnus A presented in Figure 1. Panel a shows the variations resulting from a Gaussian-shaped enhancement in the electron density with a maximum phase change of 1.7 × 10⁹ rad Hz (corresponding to an enhancement of 2 × 10¹¹ m⁻³ across an altitude range of 10 km) and a standard deviation of 1 km. The scattering screen was assumed to be at an altitude of 120 km, moving with a velocity of 22 m s⁻¹ and the radio source was assumed to be at an elevation of 34°. Panels b-g show the effects of varying some of these assumptions by changing one parameter from those used for panel a in each plot. The phase change was halved to 8.4 × 10⁸ rad Hz in panel b and doubled to 3.4 × 10⁹ rad Hz in panel c. The standard deviation of the Gaussian was doubled to 2 km in panel d and halved to 500 m in panel e. The velocity of the scattering screen was halved to 11 m s⁻¹ in panel f and doubled to 44 m s⁻¹ in panel g. The timescale and the intensity scale vary between panels. The maximum intensity is indicated in the bottom left of each panel. White represents a relative intensity of 1.0 in every panel.