The X wave is the "extraordinary" wave because it has a more complicated dispersion relation. It is partly transverse (with E1⊥B0) and partly longitudinal. As the density is increased, the phase velocity rises from c until the cut-off at ωR is reached. As the density is further increased, the wave is evanescent until the resonance at the upper hybrid frequency ωh. Then it can propagate again until the second cut-off at ωL. The cut-off frequencies are given by
where is the electron cyclotron resonance frequency, and is the electron plasma frequency.
Read more about this topic: Electromagnetic Electron Wave
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