A plasma wave is a collective disturbance of charged particles and electromagnetic fields in a plasma. Restoring forces can involve charge separation, magnetic tension or pressure, producing different branches rather than one universal wave speed.
For a cold unmagnetized electron fluid with effectively stationary neutralizing ions, electrostatic density oscillations have electron plasma frequency . Displacing electrons creates an electric field that pulls them back; inertia produces oscillation. Measuring that frequency therefore constrains electron density. Thermal motion, magnetic fields and collisions modify the dispersion and damping, and must be included when the cold-fluid assumptions fail.
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