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VOLUME 22, NUMBER 15 PHYSICAL REVIEW LETTERS 14APRIL1969 and the linearized Boltzmann equation solved for the perturbed velocity distribution. The current density can then be calculated and related back to the electric field by means of Maxwell's equations so determining the coefficients a n . For a) «w. and a Maxwellian velocity distribution in the unperturbed state,where B(a) is the amplitude of the magnetic field at x = ±a, c 0 is the most probable speed of electrons, Z(|) is the plasma dispersion function, 7 and £ zz 2av(i-(x)/v)('nc ( fi)~1. The electromagnetic field and the current distributions are then determined, leading to a value for R. In making these calculations we put i>= ^en + ^ei> where the electron-ion collision frequency u e i was obtained from an expression for the resistivity of a fully ionized plasma given by Spitzer. 8 The theory confirms the presence of a critical frequency and density for which R is much greater than predicted by cold plasma theory. Theoretical values of R are shown in Fig. 1 for a frequency near the critical value. Despite the plane geometry used in the theoretical treatment, the theory gives a good description of the observed field penetration. On the other hand, the coldplasma theory in plane geometry giveswhich is plotted in Fig. 1 for the same value of

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