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Scintillation spectra of some pulsars have suggested the existence of$\lesssim$ AU scale density structures in the ionized interstellar medium,whose astrophysical correspondence is still a mystery. The detailed study ofBrisken et al. suggested two possible morphologies for these structures: aparallel set of filaments or sheets (the `parallel stripes model'), or afilament broken up into denser knots (the `threaded beads model'). Here wepropose a straightforward test that can distinguish these two morphologies:whether the apex of the main parabolic arc created by the scattered imagesdeviates from the origin of the scintillation spectrum or not. In the `parallelstripes' model, the scattered images move along the stripes as the relativeposition of the pulsar moves. As a result, the pulsar is always co-linear withthe scattered images, and thus the apex of the main parabolic arc stays at theorigin of the scintillation spectrum. In the `threaded beads' model, thescattered images remain at almost fixed positions relative to the densitystructures, and thus the pulsar is not co-linear with the scattered images atmost times, leading to an offset between the apex and the origin. Looking forthis possible offset in a large sample of pulsar scintillation spectra, ormonitoring the evolution of parabolic arcs will help pin down the morphology ofthese tiny density structures and constrain their astrophysical origin.

Morphology of solar system scale plasma lenses in the interstellar medium: a test from pulsar scintillation parabolic arcs