  Pulsar dynamic spectra sometimes show organised interference patterns; these
patterns have been shown to have power spectra which often take the form of
parabolic arcs, or sequences of inverted parabolic arclets whose apexes
themselves follow a parabolic locus. Here we consider the interpretation of
these arc and arclet features. We give a statistical formulation for the
appearance of the power spectra, based on the stationary phase approximation to
the Fresnel-Kirchoff integral. We present a simple analytic result for the
power-spectrum expected in the case of highly elongated images, and a
single-integral analytic formulation appropriate to the case of axisymmetric
images. Our results are illustrated in both the ensemble-average and snapshot
regimes. Highly anisotropic scattering appears to be an important ingredient in
the formation of the observed arclets.

Astrophysics