Case studies on the wave propagation and polarization of ELF emissions observed by Freja around the local proton gyrofrequency
Abstract
Using a multicomponent measurement of electric and magnetic fields in the ELF range, we investigate the polarization and propagation characteristics of several types of electromagnetic emissions around the local proton gyrofrequency. The data of the loworbiting Freja satellite are examined in the auroral and subauroral regions as well as inside the plasmasphere. We confirm previous results on the properties of the high-latitude hiss with a sharp lower cutoff just below the local proton gyrofrequency. The waves propagate downward from higher altitudes outside the plasmasphere and reflect at the local two-ion cutoff frequency. With an enhanced data representation we are able to characterize the reflected upgoing waves on both poleward and equatorward edges of the hiss emission. The high-latitude emissions in a large band below the local proton gyrofrequency contain right-hand circularly polarized waves propagating nearly parallel to the terrestrial magnetic field. We believe these emissions can originate by tunnelling of the right-hand-polarized hiss below the two-ion crossover frequency. We report high-latitude "frequency drift phenomena" below the local proton gyrofrequency. The frequency of these faint emissions increases by several tens of hertz per latitude degree. The electromagnetic noise at low frequencies is always found with a high degree of a nearly linear polarization. Upgoing ion whistlers are observed to follow each other with time delays as low as about 100 ms.
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