Noodle Models for Scintillation Arcs. (arXiv:1811.05996v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Gwinn_C/0/1/0/all/0/1">Carl R. Gwinn</a>
I show that multiple, 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
less-strongly-scattered pulsars. The noodle model assumes that the strips are
extremely long, with widths comparable to a pair of Fresnel zone at their
separation from the origin. Their lengths are many times a Fresnel scale, or
hundreds or thousands times their widths. Physically, the strips may correspond
to filaments or sheets of over- or under-dense plasma, with a normal
perpendicular to the line of sight. They may lie in reconnection sheets, along
magnetic field lines. If so, observations of scintillation arcs would allow
visualization of magnetic fields in reconnection regions. Along the strips, the
Kirchhoff integral 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 to the observer plane. Most observations can measure for each strip
only an amplitude, that is related to the width of the strip and its phase
contrast with surrounding material, and a phase, that varies with the geometric
phase where the strip is closest to the line of sight. Observations suggest a
minimum strip width of about 800 km, comparable to the ion cyclotron radius.
I show that multiple, 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
less-strongly-scattered pulsars. The noodle model assumes that the strips are
extremely long, with widths comparable to a pair of Fresnel zone at their
separation from the origin. Their lengths are many times a Fresnel scale, or
hundreds or thousands times their widths. Physically, the strips may correspond
to filaments or sheets of over- or under-dense plasma, with a normal
perpendicular to the line of sight. They may lie in reconnection sheets, along
magnetic field lines. If so, observations of scintillation arcs would allow
visualization of magnetic fields in reconnection regions. Along the strips, the
Kirchhoff integral 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 to the observer plane. Most observations can measure for each strip
only an amplitude, that is related to the width of the strip and its phase
contrast with surrounding material, and a phase, that varies with the geometric
phase where the strip is closest to the line of sight. Observations suggest a
minimum strip width of about 800 km, comparable to the ion cyclotron radius.
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