Particle‐in‐Cell Simulations of a Nonlinear Transverse Electromagnetic Wave in a Pulsar Wind Termination Shock

A 2.5-dimensional particle-in-cell code is used to investigate thepropagation of a large-amplitude, superluminal, nearly transverseelectromagnetic (TEM) wave in a relativistically streaming electron-positronplasma with and without a shock. In the freestreaming, unshocked case, theanalytic TEM dispersion relation is verified, and the streaming is shown tostabilize the wave against parametric instabilities. In the confined, shockedcase, the wave induces strong, coherent particle oscillations, heats theplasma, and modifies the shock density profile via ponderomotive effects. Thewave decays over $\gtrsim 10^2$ skin depths; the decay length scale dependsprimarily on the ratio between the wave frequency and the effective plasmafrequency, and on the wave amplitude. The results are applied to thetermination shock of the Crab pulsar wind, where the decay length-scale (atleast 0.05") might be comparable to the thickness of filamentary, variablesubstructure observed in the optical and X-ray wisps and knots.Comment: 10 pages, 4 figures. Accepted for publication in ApJ (2005