Simultaneous Auroral Cyclotron Harmonic Radio Emissions

Auroral roar originates in Earth’s ionosphere at altitudes of several hundred kilometers where the upper hybrid frequency matches a harmonic of the electron gyrofrequency. These radio emissions are important for remote sensing of ionospheric plasma conditions and processes, and their physics is similar to that of astrophysical radio emissions. In this study, direction finding was used to establish the distribution of direction of arrival (DOA) angles for the third harmonic (3fce) emissions and to compare the direction angles of second harmonic (2fce) and 3fce events when they occur simultaneously. DOA data were collected from May 9, 2022 to May 20, 2023 by a three antenna array in Toolik Lake, AK (68.6° N, 149.6° W, 68.5°magnetic latitude). The interferometer DOA was checked using local AM radio stations. A tool was created to select auroral events from spectrograms. After selecting the events of interest users can select and remove interference lines, and filter by power, frequency, azimuth, and elevation. 73 examples of simultaneous 2&3fce auroral cyclotron harmonic emissions were observed. 31 were discarded because the power was too weak to enable a reliable DOA measurement. The 3fce emissions are susceptible to spatial aliasing that can occur when the wavelength of the incoming signal, projected onto the plane of the measurements, exceeds the distance between any two pairs of receivers. This can lead to an ambiguous interpretation of the phase difference which in turn leads to ambiguity in the calculated DOA. To simulate the aliasing and to determine the ranges of DOA affected by it, simulated signals arising from all unique combinations of angles were created using the specific separation distance of the three-antenna array, and average frequencies of each event. These simulations were used to correct the aliased DOA measurements. About ¼ of the 42 events were on the same azimuth within 10°. This is a lower bound on the fraction of simultaneous harmonic 2&3fce emissions that come from the same auroral arc. This subset predominantly supports the ionospheric density profile theory which says that 2fce emissions sourced from the bottom side of the profile will have a lower elevation angle than 3fce most of the time. The events that did not agree with this theory can be explained by refraction, or could be sourced from the top side.