Scattering of pulsar radio emission by the interstellar plasma
  We present simulations of scattering phenomena which are important in pulsar
observations, but which are analytically intractable. The simulation code,
which has also been used for solar wind and atmospheric scattering problems, is
available from the authors. These simulations reveal an unexpectedly important
role of dispersion in combination with refraction. We demonstrate the effect of
analyzing observations which are shorter than the refractive scale. We examine
time-of-arrival fluctuations in detail: showing their correlation with
intensity and dispersion measure; providing a heuristic model from which one
can estimate their contribution to pulsar timing observations; and showing that
much of the effect can be corrected making use of measured intensity and
dispersion. Finally, we analyze observations of the millisecond pulsar
J0437$-$4715, made with the Parkes radio telescope, that show timing
fluctuations which are correlated with intensity. We demonstrate that these
timing fluctuations can be corrected, but we find that they are much larger
than would be expected from scattering in a homogeneous turbulent plasma with
isotropic density fluctuations. We do not have an explanation for these timing
fluctuations.
