Propagation analysis of electromagnetic waves between the helium and proton gyrofrequencies in the low-altitude auroral lone

We present a statistical study of electromagnetic emissions with a lower frequency limitation observed in a broadband below the local proton gyrofrequency. An extensive data set recorded by the low-orbiting Freja satellite is examined. The emissions occur outside the plasmasphere, mainly at auroral latitudes. The maximum wave power is observed before the local noon, and no events have been detected during the local night. In the majority of cases we can assume the presence of a single plane wave. Further analysis shows a nearly circular and right-handed polarization. The statistics of wave vector directions reveal a downward and nearly field aligned propagation. An experimental estimation of the phase velocity modulus is in rough agreement with the cold plasma theory. The origin of the emissions could be possibly explained by the tunnelling of the downgoing ELF hiss below the two-ion cross-over frequency.