  The interstellar scattering responsible for pulsar parabolic arcs, and for
intra-day variability of compact radio quasars, is highly anisotropic in some
cases. We numerically simulate these observed phenomena using totally
anisotropic, power-law models for the electron density fluctuations which cause
the scattering. By comparing our results to the scattered image of PSR B0834+06
and, independently, to dual-frequency light curves of the quasar PKS1257-326,
we constrain the nature of the scattering media on these lines of sight. We
find that models with spectral indices slightly below \beta=3, including the
one-dimensional Kolmogorov model, are broadly consistent with both data sets.
We confirm that a single physical model suffices for both sources, with the
scattering medium simply being more distant in the case of B0834+06. This
reinforces the idea that intra-day variability and parabolic arcs have a common
cause in a type of interstellar structure which, though obscure, is
commonplace. However, the implied gas pressure fluctuations are large compared
to typical interstellar pressures, and the magnetic stresses are much larger
still. Thus while these scattering media may be commonplace, their underlying
dynamics appear quite extraordinary.
