Measurement of the Plasma Density Using the Intensification ofz-Mode Waves at the Electron Plasma Frequency

A new method is proposed for measuring the number density of a plasma using spectral measurements of plasma waves. The method is based on predictions that the ratio of electric to magnetic fields and the electric intensity is significantly enhanced in a narrow frequency band about the electron plasma frequency for $z$-mode waves propagating at small angles to the background magnetic field in cold relatively strongly magnetized plasmas. The method is applied to emissions observed in space over Earth's polar cap, allowing identification of the plasma frequency and the wave mode consistent with the data available. This method should be useful for other cold, strongly magnetized plasmas in space and in the laboratory.