Spectral interpretation of radio-sounder-stimulated magnetospheric plasma resonances in terms of kappa distributions

Ionospheric topside sounders stimulate a wide variety of signal returns, which have been labeled plasma resonances, including two sequences that appear between the harmonics of the electron cyclotron frequency f_ce. One, known as the Qn sequence, occurs above the upper-hybrid frequency f_uh, where f_uh² = f_ce² + f_pe² and f_pe is the electron plasma frequency; this sequence has been attributed to slowly-propagating electrostatic Bernstein-mode waves [1, 2]. Similar resonances are stimulated by magnetospheric radio sounders. In this case, however, it is often necessary to introduce a non-Maxwellian electron-velocity distribution function into the dispersion equation used to calculate the Qn frequencies to obtain agreement with observations [3]. Interpretations based on bi-Maxwellian [3] and kappa [4] distributions have been proposed. Here we expand on the latter, which requires fewer free parameters, by comparing parametric curves of kappa-derived Qn frequencies with observations from the Radio Plasma Imager (RPI) on the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) satellite. The calculated Qn frequencies based on a Maxwellian distribution are higher than those based on a kappa distribution and the differences have been found to approach 20% of the ambient f_ce value [4]. Applying this radio spectrometry to IMAGE/RPI plasmagrams, after accurate f_pe/f_ce determinations have been obtained, enables the determination of electron-velocity-distribution function information pertaining to the ambient magnetospheric plasma. Such accurate f_pe/f_ce determinations can best be made when a sounder generated wave cutoff can be determined in addition to identifying and determining the frequencies of sounder-stimulated plasma resonances. We will demonstrate this procedure using IMAGE/RPI data.