Estimating Polar Cap Density and Medium‐Frequency Burst Source Heights Using 2fce Roar Radio Emissions

Abstract

AbstractCompeting 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 2fce roar 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 2fce roar 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 the E region rather than the F region. 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.