Electromagnetic bounded States and challenges of plasma spectroscopy

We review the foundations and observational support for a new plasma wave mode that has been proposed to explain emissions stimulated by satellite-born high-power radio-frequency sounders. These sounders, which are designed to derive remote electron density profiles from electromagnetic-wave echoes, stimulate plasma resonances in the local plasma. In addition to stimulating resonances at the electron plasma frequency f/sub pe/, harmonics of electron gyrofrequency f/sub ce/, and at the upper hybrid frequency f/sub uh/, resonances are also stimulated at frequencies below f/sub pe/ between the gyroharmonics. The first such resonance, with frequency between f/sub ce/ and 2f/sub ce/, was discovered by Nelms and Lockwood in 1967 using the topside sounder on Alouette 2. From its appearance, it was called a diffuse resonance (D/sub 1/ in modern notation) and was originally thought to have a frequency of 3/2f/sub ce/. Further observations, and theoretical insights, indicated that f/sub D1/ is dependent on f/sub pe/ and f/sub ce/, namely, f/sub D1/=(3//spl pi/)(f/sub pe/f/sub ce/)/sup 1/2/. The theoretical concepts used to obtain this expression indicated that the D/sub n/ resonances (n=1,2,3,...) are electron eigenmodes (electromagnetic bounded states) and led to equations describing the frequencies of the entire D/sub n/ sequence and their subsidiary resonances D/sub n//sup +/ and D/sub n//sup -/. Such formulas, based on a combined theoretical and empirical approach, describe a /spl radic/n spectrum with the frequency of each mode split by the magnetic field analogous to the Zeeman splitting in quantum mechanics. The /spl radic/n spectrum was first predicted for force-free electromagnetic cylindrical plasma oscillations by Osherovich in 1986. The main theoretical challenge remains to derive the frequency f/sub D1/ from first principles. Although the D/sub n/ resonances have been stimulated in the terrestrial ionosphere for decades by topside sounder satellites such as Alouette 2 and by electron guns on several rocket experiments, and in Jupiter's Io plasma torus by the sounder on the Ulysses space probe, and are currently being stimulated in the terrestrial magnetosphere by the sounder on the IMAGE spacecraft, they have yet to be observed in laboratory experiments.