Collaborative Research: Direction-Finding Measurements of LF/MF/HF Auroral Radio Emissions at South Pole

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The proposed project is focused on improving the experimental technique currently used at South Pole for better directional detection and understanding of the sources and propagation of observed low, medium, and high frequency (50 - 5000 kHz) emissions (and specifically medium-frequency bursts) generated in the auroral ionosphere by the solar and geomagnetic activity. The studies of near-Earth electromagnetic environment is important because ionospheric plasma waves and instabilities play a direct role in mediating exchange of the energy between charged particle populations, provide effective means of remote sensing of the ionospheric and magnetospheric plasma processes and boundaries, and represent a "local laboratory" for studying emissions mechanisms which also occur in remote planetary or astrophysical plasmas. The low-noise electromagnetic environment of the South Pole Station in the above-mentioned frequency band allows to study statistically two auroral radio emissions: (a) auroral roars, a narrowband emission near the electron cyclotron harmonics; and (2) auroral MF bursts, a broadband emission associated with geomagnetic substorm onsets. A versatile waveform receiver (deployed at South Pole since 2002) captured scores of auroral roar emissions at full resolution, and numerous examples of fully resolved low-frequency auroral hiss events are now available for the study. It is proposed to complete the analysis of the high-resolution "auroral roar" and "hiss" data, but then focus on the auroral medium-frequency (MF) bursts - an outstanding mystery of the aurora because its generation mechanism is still unknown. The project plans to deploy a new digital direction-finding receiver at South Pole and use it to perform a few significant observational experiments: (1) identify features of the substorm expansion correlated with the MF bursts by tracking the azimuth of bursts' arrival as a function of time as the substorm evolves; (2) compare and validate different methods of determining substorm onsets by comparing the timing from ultra-low-frequency and optical data and MF bursts; (3) analyze multi-wavelength optical and imaging riometer data to determine characteristics of the auroral electron spectra that cause the MF burst; and (4) determine the detailed relationship between MF burst and auroral hiss emissions using digital data from perpendicular crossed antennas and the handedness of polarization (indicative of the propagation mode) of MF burst events extending below 1500 kHz. The first two experiments will help discovering new experimental methods for the substorm studies, and the rest - to focus on determining the source mechanism of the MF bursts and its relation to other radio emissions. These experiments will provide significant experimental materials for theorists to markedly advance our knowledge of the auroral radio emissions and their relation to macrophysics of various auroral phenomena..