>100 Hz Periodicities in Low Frequency Auroral Hiss

Abstract:

In recent years, waveform measurements of natural auroral radio emissions at frequencies up to 2000 kHz have been made with two perpendicular loop antennas at South Pole Station, Antarctica. Signals received include Auroral Kilometric Radiation, Medium Frequency Burst, and Low- and Medium-Frequency (LF and MF) Auroral Hiss. During austral winter 2019, waveforms of LF/MF auroral hiss at frequencies up to and exceeding 200 kHz were recorded for 134 events between 11-June and 03-October. Automated software was developed to search for >1-Hz periodicities in these hiss events at frequencies 100-200 kHz which is a particularly clear band at South Pole. The spectrum of the variations in auroral hiss power usually extends to a few tens of Hz and is broadband without distinct peaks. However, on two occasions on 01-September, 2019, peaks occurred near 150 Hz lasting 10-20 seconds. These consisted of coherent rising tones taking approximately 4 ms to cover a 100-kHz band. Re-examination of the larger data set revealed examples of this phenomenon on several other days, though with shorter durations. Flickering aurora typically has frequencies tens of Hz but has occasionally been recorded at frequencies as high as 180 Hz [e.g., McHarg et al., 1998] Variations in auroral hiss might be expected in association with flickering aurora; however, since hiss may arrive at the receiving station from aurora occurring over a wide area, it is not surprising that periodic signatures resulting from relatively localized regions of flickering are rare. Although rare, these events are significant because they enable testing of models of auroral hiss generation and propagation. (References: McHarg, M.G., D.L. Hampton, and H.C. Stenbaek-Nielsen: 1998, Fast photometry of flickering in discrete auroral arcs, Geophys. Res. Lett., 25, 2637, 10.1029/98GL01972.)