A Model for the Moving “Wisps” in the Crab Nebula

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Abstract
I propose that the moving "wisps" near the center of the Crab Nebula result from nonlinear Kelvin-Helmholtz instabilities in the equatorial plane of the shocked pulsar wind. Recent observations suggest that the wisps trace out circular wave fronts in this plane, expanding radially at speeds c/3. Instabilities could develop if there is sufficient velocity shear between a faster moving equatorial zone and a slower moving shocked pulsar wind at higher latitudes. The development of shear could be related to the existence of a neutral sheet—with a weak magnetic field—in the equatorial zone, and could also be related to a recent suggestion in the literature that the magnetic field in the Crab pulsar wind is much stronger than had been thought. I show that plausible conditions could lead to the growth of instabilities at the radii and speeds observed, and that their nonlinear development could lead to the appearance of sharp wisplike features.
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