Description
Understanding the kinetic properties of strongly nonlinear electromagnetic waves in highly magnetized pair plasmas is essential for establishing the connection between radiation produced near neutron stars and the radiation that ultimately escapes and is observed. In this work, we identify and analyse a previously unaccounted, to the best of our knowledge, nonlinear process affecting X-mode waves with a large nonlinearity parameter $a_0\ll 1$. As these waves propagate through a highly magnetized cold pair plasma, they undergo a self-similar cascade that excites higher harmonics, leading to a modification of the electromagnetic wave structure. We develop an analytical model describing this process and derive the associated scalings, which are validated using large-scale particle-in-cell (PIC) simulations. PIC simulations are further employed to investigate the nonlinear late-time evolution of the wave packet. The resulting wave spectra exhibit a clear power-law structure, with potential implications for the ability of coherent radiation to escape the inner and outer magnetospheres of neutron stars, and how such radiation is observed.
PIC simulations were performed on Marenostrum-5 under the MAPs (Masers in Astrophysical Plasmas)-2 project, allocation 2024.11062.CPCA.A3.