Direction Finding Studies of Simultaneous Auroral 2fce and 3fce Roar Cyclotron Harmonic Radio Emissions

Abstract

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 $\left(3f_{ce}\right)$ emissions and to compare the direction angles of second harmonic $\left(2f_{ce}\right)$ and $3f_{ce}$ events when they occur simultaneously. Data were collected for 42 events from 9 May 2022 to 20 May 2023 by a three antenna array in Toolik Lake, AK (68.6°N, 149.6°W, 68.5° magnetic latitude) with a DOA distribution centered overhead. 30% of the events coming from the south, for which azimuth deviations due to refraction are less significant, were on the same azimuth within 10°, the uncertainty of the measurement. This is a lower bound on the fraction of simultaneous harmonic $2f_{ce}$ and $3f_{ce}$ emissions that come from the same auroral arc. All events coming from the south that had a $2f_{ce}$ and $3f_{ce}$ azimuth angle of arrival within 10° had a higher $3f_{ce}$ elevation than $2f_{ce}$ elevation within experimental uncertainty, supporting the mechanism by which these emissions are excited at the “double resonance” points on the bottomside of the ionosphere.