Ground based observations of low frequency auroral hiss fine structure

An electric field waveform receiver, operated at South Pole Station during 2004, recorded numerous low‐frequency (LF) auroral hiss events, revealing fine structures which fall into three categories, in order of occurrence rate: patchy/continuous features, which have wide bandwidth (100s of kHz) and slow time variation (of order 1 s or longer); vertical/impulsive features, which have wide bandwidth (100s of kHz) and short duration (≪1 s); and discrete features, which have narrow bandwidths (100s of Hz to a few kHz) and a range of durations. We investigate two possible generation mechanisms for the discrete features. First, they may arise from mode conversion of narrowband Langmuir waves generated where the discrete feature frequency matches the plasma frequency in the auroral ionosphere. In fact, the estimated source power of narrowband discrete whistler modes observed in recent sounding rocket experiments and the estimated source power of the discrete features observed at South Pole station are ordered in the manner expected if mode conversion of Langmuir waves accounts for each of these observations. Second, Sonwalkar and Harikumar (2000) propose that auroral hiss must mode convert on meter‐scale density irregularities in order to gain access to ground level; we put forth that if the meter‐scale irregularities are highly monochromatic, the resulting mode‐converted whistler modes that reach the ground will be composed of narrow‐band discrete features. Ray‐tracing studies in a model auroral ionosphere suggest that scattering from 10–100 m irregularities could account for the fine structures in the LF auroral hiss observed at South Pole Station.