A variety of nonlinear effects occur in a plasma. First, there are the
wave- steepening effects which can occur in any fluid in which the
propagation speed depends upon the wave-amplitude. In a dispersive
medium this can lead to classes of nonlinear waves which may have
stationary solutions like solitons and shocks. Because the plasma also
acts like an inherently nonlinear dielectric resonant interactions among
waves lead to exchange of energy among them. Further, an electromagnetic
wave interacting with a plasma may parametrically excite other waves in
the plasma. A large-amplitude Langmuir wave undergoes a modulational
instability which arises through local depressions in plasma density and
the corresponding increases in the energy density of the wave electric
field. Whereas a field collapse occurs in two and three dimensions, in a
one-dimensional case, spatially localized stationary field structures
called Langmuir solitons can result. Many other plasma waves like
upper-hybrid waves, lower-hybrid waves etc. can also undergo a
modulational instability and produce localized field structures. A new
type of nonlinear effect comes into play when an electromagnetic wave
propagating through a plasma is strong enough to drive the electrons to
relativistic speeds. This leads to a propagation of an electromagnetic
wave in a normally overdense plasma, and the coupling of the
electromagnetic wave to a Langmuir wave in the plasma. The relativistic
mass variation of the electrons moving in an intense electromagnetic
wave can also lead to a modulational instability of the latter.