Absorption and Reflection Spectrum of a Plasma

A series of absorption and reflection peaks for plasmas, cylindrical in form, and occurring at electron densities lower than that required for the conventional plasma frequency, has been reported by various workers such as Tonks, Rommel, and Dattner. This paper describes experiments at a wavelength of 10 cm in which the multiple resonances are observed both within a wave guide and in free space. The electric field of the incident microwave is perpendicular to the axis of the plasma column. The positions of the absorption peaks are given by the interference-dispersion relationship, 2n/(2n+1)=[1−(f0/f)2]12. The mode spectrum is found by assigning integral values, including zero, to n. When n=0, we obtain the conventional plasma frequency f0, but for n>0, the frequencies, and electron densities for higher order modes are obtained. The relationship identifies a distinctive set of ratios between the phase velocity of a plasma wave and the rms velocity of the electrons such that simultaneously the conditions are met for the existence of a standing plasma wave and a stationary pattern of electron motion. Application of the relationship to radar echoes from meteor trails is indicated.