Small-scale variations of HI spectra from interstellar scintillation

I suggest that radio-wave scattering by the interstellar plasma, in combina tion with subsonic gradients in the Doppler velocity of interstellar H I, i s responsible for the observed small-scale variation in H I absorption spec tra of pulsars. Velocity gradients on the order of 0.05-0.3 km s(-1) across 1 AU can produce the observed variations. I suggest observational tests to determine the relative importance of such gradients and AU-scale absorbing cloudlets in producing the spectral variations.