Predicting Pulsar Scintillation from Refractive Plasma Sheets
  The dynamic and secondary spectra of many pulsars show evidence for
long-lived, aligned images of the pulsar that are stationary on a thin
scattering sheet. One explanation for this phenomenon considers the effects of
wave crests along sheets in the ionized interstellar medium, such as those due
to Alfv\'en waves propagating along current sheets. If these sheets are closely
aligned to our line-of-sight to the pulsar, high bending angles arise at the
wave crests and a selection effect causes alignment of images produced at
different crests, similar to grazing reflection off of a lake. Using geometric
optics, we develop a simple parameterized model of these corrugated sheets that
can be constrained with a single observation and that makes observable
predictions for variations in the scintillation of the pulsar over time and
frequency. This model reveals qualitative differences between lensing from
overdense and underdense corrugated sheets: Only if the sheet is overdense
compared to the surrounding interstellar medium can the lensed images be
brighter than the line-of-sight image to the pulsar, and the faint lensed
images are closer to the pulsar at higher frequencies if the sheet is
underdense, but at lower frequencies if the sheet is overdense.
