Inhomogeneities of refractive index in a thin screen, mid way between source and observer
Essentially a Young's double slit experiment, but with multiple slits
An interference pattern forms
Earth and pulsar motion cause different parts of the interference
pattern to be sampled
Results in random flux variations
Interference pattern is frequency dependent
Figure 11: The thin screen model. From Lyne & Graham-Smith 1990
[23]
Wavelength scale of amplitude variations is 
In practice the screen has thickness D and the frequency bandwidth of
amplitude variations is
Figure 12: Amplitude variations of PSR B0329+54 as a function of time
as observed using MERLIN