Small-Scale Variations of HI Spectra from Interstellar Scintillatio

I suggest that radio-wave scattering by the interstellar plasma, in combination with subsonic gradients in the Doppler velocity of interstellar HI, is responsible for the observed small-scale variation in HI absorption spectra of pulsars. Velocity gradients on the order of 0.05 to 0.3 km/s across 1 AU can produce the observed variations. I suggest observational tests to distinguish between this model and the traditional picture of small-scale opacity variations from cloudlets.