Relativistically-strong electromagnetic waves in magnetized plasmas

Using a two-fluid approach, we consider the properties of relativistically nonlinear (arbitrary $a_0$), circularly polarized \EM\ waves propagating along magnetic field in electron-ion and pair plasmas. Dispersion relations depend on how wave intensity scales with frequency, $a_0 (\omega)$. For superluminal branches, the nonlinear effects reduce the cut-off frequency, while the general form of the dispersion relations $\omega(k)$ remains similar to the linear case. For subluminal waves, whistlers and Alfven, a new effect appears: dispersion curves effectively terminate at finite $\omega^\ast - k^\ast$, where the group velocity becomes zero. Qualitatively, subluminal modes with fluctuating electric field larger than the guide field, $E_w (\omega) \geq B_0$, cannot propagate. In extended systems, e.g., within magnetospheres of neutron stars, this leads to opening of the magnetosphere by a strong wave.