Statistical study of ELF/VLF emissions at subauroral latitudes in Athabasca, Canada

Abstract

We present the first statistical analysis of ELF/VLF emissions observed on the ground at subauroral latitudes that includes their features, occurrences, and association with solar wind and geomagnetic variations. Using a 100 kHz sampling loop antenna located in Athabasca, Canada (54.60N, 246.36E, L = 4.3), we monitored these emissions, including chorus, quasiperiodic emissions, and hiss, from November 2012 to October 2013. We found a maximum occurrence rate in the morning sector (06-07 MLT, magnetic local time) and a minimum in the night sector (18 to 02 MLT), in agreement with previous satellite measurements in the inner magnetosphere. We also found correlation between the ongoing substorm and storm activity and the increase of occurrence rates. The observed waves usually had a central frequency 1-3 kHz lower than the half-gyrofrequency at the conjugate equatorial plane, indicating a wave source at higher latitudes. A superposed epoch analysis showed that the starting time of the ELF/VLF emissions is preceded by a rise in AE both on short (hours) and long (days) terms. Solar wind speed also started slowly rising 1.5 days before, while density and dynamic pressure decreased shortly afterward. This may signify that high-speed solar wind conditions also contribute to the generation of ELF/VLF emissions detected at subauroral latitudes.