Siddhant Solanki, Jens Mahlmann, Alexander Philippov

We investigate the propagation of low-frequency fast-magnetosonic (FMS) waves in highly magnetized environments. Such conditions are relevant to the escape of GHz fast radio bursts potentially produced in the inner magnetospheres of magnetars. It remains an open question whether such waves can escape without substantial reprocessing. Using relativistic force-free electrodynamics simulations, we confirm the key theoretical predictions of Golbraikh & Lyubarsky (2023) and demonstrate that FMS waves undergo efficient nonlinear conversion into secondary FMS and Alfvén waves via the parametric decay instability. This process continues to drain energy from the primary FMS waves even after approximate energy equipartition between the FMS and Alfvén components is established. The resulting spectrum of excited waves is broad, extending across much of the inertial range in $k$-space within the simulation domain. Our results indicate that FMS waves likely do not escape magnetar magnetospheres without substantial dissipation and spectral broadening.