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For certain values of the plasma parameters, goes to zero
or infinity. In both cases, a transition is made from a region
of propagation to a region of evanescense, or vice versa.
It will be demonstrated later on that reflection occurs
wherever goes through zero, and that absorption takes place
wherever goes through infinity. The former case is called a
wave cutoff, whereas the latter case is termed a wave resonance.
According to Eqs. (492) and (493)(495), cutoff occurs when

(509) 
or

(510) 
or

(511) 
Note that the cutoff points are independent of the direction of
propagation of the wave relative to the magnetic field.
According to Eq. (498), resonance takes place when

(512) 
Evidently, resonance points do depend on the direction of propagation
of the wave relative to the magnetic field. For the case of
parallel propagation, resonance occurs whenever
.
In other words, when

(513) 
or

(514) 
For the case of perpendicular propagation, resonance takes place
when

(515) 
Next: Waves in an Unmagnetized
Up: Waves in Cold Plasmas
Previous: Polarization
Richard Fitzpatrick
20110331