| A VersatIle Electromagnetic Waveform (VIEW) receiver was deployed at South Pole station in December 2002 in order to measure the fine structure of auroral radio emissions with high resolution and an unprecedented 500--1000 kHz bandwidth. Using this new instrument and semi-automatic data acquisition software, we greatly expanded our database of flickering auroral roar, auroral roar fine structure and LF auroral hiss fine structure. Based on 10 events captured at South Pole in 2003, the flickering frequency of auroral roar is found to range from 3 to 30 Hz, which compares favorably to the flickering frequency of optical aurora and the modulation frequency of the precipitating auroral electrons. The 2003 South Pole data set also greatly expanded the database of the fine frequency structure of auroral roar, which is currently explained by eigenmodes of upper hybrid waves trapped inside a density enhancement. Two methods for calculating the eigenfrequencies were compared, and a database of eigenfrequencies was created using the more exact full numerical method. The HIBAR rocket, launched into an active auroral arc in Alaska in 2003, detected banded structures in upper hybrid waves, the source of auroral roar. A fitting method was developed to test the eigenmode theory with these in-situ measurements. We then applied the same fitting method to the 2003 South Pole data set in a statistical study of the density enhancement structures causing auroral roar fine structures. The 2004 South Pole data set revealed for the first time LF auroral hiss fine structure, which was categorized into three types: patchy/continuous features, vertical/impulsive features and discrete features. We proposed two generation mechanisms for the discrete features: the Langmuir wave hypothesis and the scattering hypothesis. Estimations of the wave power of whistler mode waves observed by PHAZEII and SIERRA and discrete features observed at South Pole are ordered in a manner expected if mode conversion of Langmuir waves accounts for these emissions. Ray tracing studies confirmed that auroral hiss, generated on the resonance cone, needs to be scattered by meter scale density irregularities in order to reach ground level. The wavelengths of such irregularities are estimated using a ray-tracing tool with a k-vector matching condition for wave-wave interaction. It is found that if the density irregularities are highly monochromatic, the resulting mode-converted whistler modes that reach the ground will be composed of narrow-band discrete features. |
