Consider an ionized gas of uniform electron density \(n\). Regard the positive
ions as a smeared-out continuous fluid which renders the gas macroscopically
neutral and through which the electrons can move without friction. Now assume
that, by some external means, each electron is shifted in the \(x\) direction by
the displacement \(\xi=\xi(x)\), a function of its initial, unperturbed location
\(x\). (a) Show from Gauss' law (8.2.1) that the resulting electric field is
\(E_{x}=n e \xi / e_{0} .(b)\) Show that each electron experiences a linear
(Hooke's law) restoring force such that when the external forces are removed,
it oscillates about the equilibrium position \(\xi=0\) with simple harmonic
motion at the angular frequency
$$
\omega_{p}=\left(\frac{n e^{2}}{\epsilon_{0} m}\right)^{1 / 2}
$$
which is known as the electron plasma frequency.
† For fuller discussion, see J. A. Ratcliffe, "The Magneto-ionic Theory and
Its Applications to the Ionosphere," Cambridge University Press, New York,
1959 ; M. A. Heald and C. B. Wharton, "Plasma Diagnostics with Microwaves,"
John Wiley \& Sons Inc., New York, 1965; I. P. Shkarofsky, T. W. Johnson, and
M. P. Bachynski, "Particle Kinetics of Plasma," Addison-Wesley Publishing
Company, Reading, Mass., \(1965 .\)