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Abstract Competing theories exist for the generation mechanism of auroral medium‐frequency burst (MFB). In an effort to constrain MFB source heights, this study analyzes 33 events in which MFB and auroral 2fceroar co‐occurred at Sondrestrom, Greenland. Using measurements from an array of receiving antennas, direction‐of‐arrival calculations indicate that in a given co‐occurrence, the elevation angle of MFB typically is higher than that of roar. Ray tracing is used to determine source heights of the MFB signals. Density profiles are obtained from the International Reference Ionosphere (IRI) and shifted in magnitude until each event's roar signals originate at heights where the frequency‐matching condition for 2fceroar generation is satisfied. This shifting method is validated using density measurements from the Sondrestrom incoherent scatter radar (ISR) facility for the two events with available ISR data. After shifting, ray tracing demonstrates that in 25 of the 33 events, burst originates at a height of about 200 km, lower than the typical altitude of peak electron density. However, ISR measurements show that the density profile is enhanced at low altitudes while MFB is observed, peaking in theEregion rather than theFregion. This finding implies that the MFB sources at 200 km are on the topside of the density peak, in a region of downward pointing density gradient, in qualitative agreement with the mechanism of MFB generation by Langmuir waves in the topside ionosphere. These results also suggest a new method of estimating density in the polar cap using roar signals to calibrate IRI profiles.
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This content will become publicly available on November 20, 2025
Direction Finding Studies of Simultaneous Auroral 2 f ce and 3 f ce 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 emissions and to compare the direction angles of second harmonic and 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 and emissions that come from the same auroral arc. All events coming from the south that had a and azimuth angle of arrival within 10° had a higher elevation than elevation within experimental uncertainty, supporting the mechanism by which these emissions are excited at the “double resonance” points on the bottomside of the ionosphere.
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- Award ID(s):
- 1915058
- PAR ID:
- 10557813
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Space Physics
- Volume:
- 129
- Issue:
- 11
- ISSN:
- 2169-9380
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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