Noodle Model for Scintillation Arcs
  I show that narrow, parallel strips of phase-changing material, or "noodles,"
generically produce parabolic structures in the delay-rate domain. Such
structures are observed as "scintillation arcs" for many pulsars. The model
assumes the strips have widths of a few Fresnel zones or less, and are much
longer than they are wide. I use the Kirchhoff integral to find the scattered
field. Along the strips, integration leads to a stationary-phase point where
the strip is closest to the line of sight. Across the strip, the integral leads
to a 1D Fourier transform. In the limit of narrow bandwidth and short
integration time, the integral reproduces the observed scintillation arcs and
secondary arclets. The set of scattered paths follows the pulsar as it moves.
Cohorts of noodles parallel to different axes produce multiple arcs, as often
observed. A single strip canted with respect to the rest produces features off
the main arc. I present calculations for unrestricted frequency ranges and
integration times; behavior of the arcs matches that observed, and can blur the
arcs. Physically, the noodles may correspond to filaments or sheets of over- or
under-dense plasma, with a normal perpendicular to the line of sight. The
noodles may lie along parallel magnetic field lines that carry density
fluctuations, perhaps in reconnection sheets. If so, observations of
scintillation arcs would allow visualization of magnetic fields in reconnection
regions.
