Direction finding of simultaneous medium frequency burst and hiss auroral radio emissions

Auroral radio emissions are important for remote sensing of magnetospheric and ionospheric plasma conditions and processes, and because they are similar to emissions generated by planetary and astrophysical sources. Auroral radio emissions include hiss, medium-frequency burst (MFB), cyclotron harmonic emissions, and auroral kilometric radiation. Auroral hiss is a VLF/LF whistler-mode emission originating at altitudes of several hundred to several thousand kilometers. Auroral MFB spans 1.5 - 4.5 MHz and is suspected to originate in the ionospheric topside. These emissions are well documented to often coincide in time, but it is unknown the degree to which they are spatially correlated. This study focuses on: 1) comparing the directions of arrival (DOA) of simultaneously occurring auroral hiss and MFB, and 2) investigating potential generation mechanisms of MFB on the basis of the altitude distribution of the sources. These two questions will be addressed using data collected over four years (2013-2017) from a two-part five antenna array system in Sondrestrom, Greenland (73.3° magnetic latitude), combined with ray tracing software. For question 1, direction finding techniques will be used to establish the DOA for simultaneously occurring auroral hiss and MFB, and to determine if these events come from the same auroral arc. Changes in the DOA of these events over time and frequency will be statistically analyzed to identify trends between these emissions. Additionally, ray tracing software will be used to estimate heights of the two sources to identify if beam re-formation is a plausible explanation for the simultaneous generation of these emissions from field-aligned electron beams. For question 2, two candidate broadband MFB generation mechanisms will be considered: Langmuir wave excitation for a range of altitudes and electron sound wave excitation at a single altitude. Ray tracing will be used to determine if there is a difference in the distribution of azimuths and elevations of arrival of the signals at ground level that would distinguish one source type from another. If such a difference is predicted from ray-tracing, measurements of MFB direction of arrival from Sondrestrom will be interpreted to determine which generation mechanism is more probable. The results will inform the use of MFB as a method of remotely sensing ionospheric conditions and processes.