Propagation of a strong fast wave in the magnetosphere of a neutron star

We studied the propagation of a strong, low frequency X-mode fast magnetosonic wave inside the magnetosphere of a neutron star. The relative strength ΔB/B of the wave grows as a function of radius before it reaches the light cylinder, and what starts as a small perturbation can grow to become nonlinear before it escapes the magnetosphere. Using first-principles Particle-in-Cell (PIC) simulations, we study in detail the evolution of the wave as it becomes nonlinear. We find that an initially sinusoidal wave becomes strongly distorted as ΔB/B approaches order unity. The wave attempts to develop a shock in each wavelength. The plasma particles drift into the shock and undergo coherent gyration in the rest of the wave, producing high-frequency components in the original fast wave. We show that in some parameter regimes, this mechanism may be able to produce FRB-like signatures.