  Stars twinkle to the eye through atmospheric turbulence, but planets, because
of their larger angular size, do not. Similarly, scintillation due to the local
interstellar medium will modulate the radio flux of gamma-ray-burst afterglows
and may permit indirect measurements of their angular sizes. The amplitude of
refractive scintillation is of order ten percent at ten gigahertz unless the
source size is much larger than the expected size, of order ten
microarcseconds. Diffractive scintillation is marginally possible, depending
sensitively on the source size, observing frequency, and scattering measure of
the interstellar medium.
