In an unmagnetized plasma, an electromagnetic electron wave is simply a light wave modified by the plasma. In a magnetized plasma, there are two modes perpendicular to the field, the O and X modes, and two modes parallel to the field, the R and L waves.
In an unmagnetized plasma in the high frequency or low density limit, i.e. for or , the wave speed is the speed of light in vacuum. As the density increases, the phase velocity increases and the group velocity decreases until the cut-off frequency where the light frequency is equal to the plasma frequency. This density is known as the critical density for the angular frequency ω of that wave and is given by
If the critical density is exceeded, the plasma is called over-dense.
In a magnetized plasma, except for the O wave, the cut-off relationships are more complex.
The O wave is the "ordinary" wave in the sense that its dispersion relation is the same as that in an unmagnetized plasma. It is plane polarized with E1||B0. It has a cut-off at the plasma frequency.
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 in 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
The R wave and the L wave are right-hand and left-hand circularly polarized, respectively. The R wave has a cut-off at ωR (hence the designation of this frequency) and a resonance at ωc. The L wave has a cut-off at ωL and no resonance. R waves at frequencies below ωc/2 are also known as whistler modes.
|vec kperpvec B_0, vec E_1>vec B_0||O wave|
|vec k>vec B_0 (right circ. pol.)||R wave (whistler mode)|
|vec k>vec B_0 (left circ. pol.)||L wave|