VLF propagation in a compressible ionosphere

This paper considers an extension of the Appleton-Hartree theory to include the effects of finite compressibility of the electron plasrna. Finite compressibility implies a finite velocity for acoustic waves in an unmagnetized plasma, and such waves also may occur in a magnetized plasma. The cases of interest here are those in which an acoustic wave and an electromagnetic wave are coupled by the application of a steady magnetic field.