  Nearby pulsar wind nebulae exhibit complex morphological features: jets,
torus, arcs and knots. These structures are well captured and understood in the
scope of global magnetohydrodynamic models. However, the origin of knots in the
inner radius of the Crab Nebula remains elusive. In this work, we investigate
the dynamics of the shock front and downstream flow with a special emphasis on
the reconnecting equatorial current sheet. We examine whether giant plasmoids
produced in the reconnection process could be good candidates for the knots. To
this end, we perform large semi-global three-dimensional particle-in-cell
simulations in a spherical geometry. The hierarchical merging plasmoid model is
used to extrapolate numerical results to pulsar wind nebula scales. The shocked
material collapses into the midplane, forming and feeding a large-scale, but
thin, ring-like current layer. The sheet breaks up into a dynamical chain of
merging plasmoids reminiscent of three-dimensional reconnection. Plasmoids grow
to a macroscopic size. The final number of plasmoids predicted is solely
governed by the inverse of the dimensionless reconnection rate. The formation
of giant plasmoids is a robust feature of pulsar wind termination shocks. They
provide a natural explanation for the inner-ring knots in the Crab Nebula,
provided that the nebula is highly magnetized.
