Direction Finding of Auroral Radio Emissions

Auroral roar occurs at several hundred kilometers in the atmosphere when the upper hybrid frequency matches the electron gyrofrequency, which coincide at various harmonics. Understanding these events is important for radio emissions on Earth and similar structures observed from astrophysical sources. This study focused on the emission directions of such ionospheric events at three times the electron cyclotron frequency, and compared the results to the directions at two times the frequency. The data used was taken in 2022 at a frequency range of 0-5 MHz from a three antenna array located near Toolik Lake, Alaska. It was expected that when the different harmonics were from the same substorm source they would have similar azimuthal directions. The elevation was expected to be higher for the third harmonic, since it comes from a higher altitude. A total of thirty-six events when the second and third harmonics occur simultaneously were analyzed. This was then narrowed down to thirteen events by eliminating those when the difference in azimuth angle between the harmonics was less than twenty-five degrees. Of these events, nine had a higher elevation for the third harmonic as expected. Further analysis of the third harmonic alone, not relative to the second and more events is needed to make a statistical conclusion. Other factors considered include that three out of the events selected were from the North. This was not expected for either harmonic, and was attributed to several possible factors. One cause could be ionospheric disturbance due to refraction in the directions measured for these events. Another cause could be angular aliasing due to the antenna spacing being too large relative to the wavelength leading to an indistinguishable phase difference in emission direction. This aliasing was only possible for the third harmonic, and a mathematical model was defined to indicate the regions where the aliasing could occur. Another possible explanation for the unexpected event directions was the selection process itself: only events when the second and third harmonic happened at the same time were considered.