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

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 2f ce 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 2f ce 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. Plain Language Summary Auroral substorms, which produce the northern lights, also emit radio waves. This study focuses on two types of auroral radio emissions, called roar and burst. Various mechanisms have been proposed to explain burst, and they disagree about its altitude of origin. This study tests these theories by determining burst source heights. An antenna array in Sondrestrom, Greenland, is used to detect roar and burst signals. Their directions of origin are calculated based on the antenna configuration, and the rays are traced backward to their origin. This ray tracing relies on electron density estimates provided by the International Reference Ionosphere (IRI). To make the IRI data more accurate, density values are shifted so that roar rays are traced to their known source altitudes. This method of shifting the IRI profile using roar is a novel way of estimating electron density and is validated using measurements from the incoherent scatter radar system in Sondrestrom. Density measurements also show that as burst occurs, the density peaks at low altitude, below 200 km, whereas burst originates around 200 km in most of the events analyzed. This result provides observational support for theories predicting that burst generation occurs above the altitude of peak electron density. Auroral "roar", also known as cyclotron harmonic emissions, is one of the best-known auroral radio emissions, first observed from ground ...

A Statistical Study of Auroral Medium Frequency Bursts and Anomalous Incoherent Scatter Radar Echoes

“…Azimuthal directions of arrival of MFB signals received at a given location tend to span a wide range (e.g., Figure 2 of N. Bunch et al. (2009), and Figure 3 of Burnett and LaBelle (2020)). Collectively, these results are consistent with a broad radiation pattern, but with propagation to ground level restricted to approximately the poleward 180° half‐space.…”