Electromagnetic bounded States and challenges of plasma spectroscopy Osherovich, V.A., Benson, R.F., Fainberg, J. 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 fpe, harmonics of electron gyrofrequency fce, and at the upper hybrid frequency fuh, resonances are also stimulated at frequencies below fpe between the gyroharmonics. The first such resonance, with frequency between fce and 2fce, was discovered by Nelms and Lockwood in 1967 using the topside sounder on Alouette 2. From its appearance, it was called a diffuse resonance (D1 in modern notation) and was originally thought to have a frequency of 3/2fce. Further observations, and theoretical insights, indicated that fD1 is dependent on fpe and fce, namely, fD1=(3 pi)(fpe/fce)^1/2. The theoretical concepts used to obtain this expression indicated that the Dn resonances (n=1,2,3,...) are electron eigenmodes (electromagnetic bounded states) and led to equations describing the frequencies of the entire Dn sequence and their subsidiary resonances Dn+ and Dn-. Such formulas, based on a combined theoretical and empirical approach, describe a radic/n spectrum with the frequency of each mode split by the magnetic field analogous to the Zeeman splitting in quantum mechanics. The 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 fD1 from first principles. Although the Dn 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. _______________ IEEE Transactions on Plasma Science, 33 (2), 599-608, 2005