Electromagnetic field propagation in neutral isotropic plasmas

Interested in electromagnetic waves inside plasmas with known dielectric properties, we prove that Courant-Hilbert fields of the focus wave mode type can propagate in a steady-state, isotropic, neutral plasma which is in fact a dielectric with the refractive index \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$n^{2}(\omega) = 1-\omega^{2}_{p}/\omega^{2}$\end{document} where \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$\omega_{p}$\end{document} is the plasma frequency. We also prove that neutral, isotropic plasmas with memory behave for harmonic plane waves as a time reversal mirror when the memory function is a decreasing exponential.