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Nearby pulsar wind nebulae exhibit complex morphological features: jets,torus, arcs and knots. These structures are well captured and understood in thescope of global magnetohydrodynamic models. However, the origin of knots in theinner radius of the Crab Nebula remains elusive. In this work, we investigatethe dynamics of the shock front and downstream flow with a special emphasis onthe reconnecting equatorial current sheet. We examine whether giant plasmoidsproduced in the reconnection process could be good candidates for the knots. Tothis end, we perform large semi-global three-dimensional particle-in-cellsimulations in a spherical geometry. The hierarchical merging plasmoid model isused to extrapolate numerical results to pulsar wind nebula scales. The shockedmaterial collapses into the midplane, forming and feeding a large-scale, butthin, ring-like current layer. The sheet breaks up into a dynamical chain ofmerging plasmoids reminiscent of three-dimensional reconnection. Plasmoids growto a macroscopic size. The final number of plasmoids predicted is solelygoverned by the inverse of the dimensionless reconnection rate. The formationof giant plasmoids is a robust feature of pulsar wind termination shocks. Theyprovide a natural explanation for the inner-ring knots in the Crab Nebula,provided that the nebula is highly magnetized.

Formation of giant plasmoids at the pulsar wind termination shock: A possible origin of the inner-ring knots in the Crab Nebula