Negative ion detection in the ionosphere from effects on ELF waves

Abstract

The theory for propagation of small-amplitude electromagnetic waves in a cold, homogeneous plasma including negative ions and immersed in a uniform, static magnetic field is developed. It is found that for longitudinal propagation each negative ion introduces a resonance at the negative ion cyclotron frequency, a concentration dependent cutoff frequency above the negative ion cyclotron frequency, and possibly a crossover frequency depending on the ion concentrations and charge-to-mass ratios. At both the cyclotron frequency and the cutoff frequency the group refractive index becomes infinite. Between these two frequencies there is a ‘nose’ frequency, for which the group refractive index is a minimum. Examples are given for a three-and five-component plasma. Application of this negative ion theory is made to propagation of negative ion whistlers in the ionosphere. It is found that for frequencies near the negative ion cyclotron frequencies the WKB approximation is valid above 300 km during the nighttime and 150 km during the daytime. Effects of collisions can be neglected above 150 km. An ideal experiment is proposed for observation of negative ion whistlers (1–1000 cps). Sample whistler, frequency-time spectrograms like those that would be observed with such an experiment are sketched. From the distinctive frequencies on these sample spectrograms, it is shown that the negative ion specie and concentration can be determined using the developed cold plasma expressions.