Noodle model for scintillation arcs

Abstract

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.