  The termination shock of a pulsar wind is located roughly where the ram
pressure matches that of the surrounding medium. Downstream of the shock, MHD
models of the diffuse nebular emission suggest the plasma is weakly magnetized.
However, the transition from a Poynting-dominated MHD wind to a
particle-dominated flow is not well understood. We discuss a solution of this
"sigma problem" in which a striped wind converts into a strong, superluminal
electromagnetic wave. This mode slows down as it propagates radially, and its
ram pressure tends to a constant value at large radius, a property we use to
match the solution to the surrounding nebula. The wave thus forms a pre-cursor
to the termination shock, which occurs at the point where the wave dissipates.
Possible damping and dissipation mechanisms are discussed qualitatively.
