Statistics of VLF/ELF emissions at subauroral latitudes in Athabasca, Canada and their correspondence to the Van Allen Probes observations.

Abstract:

Using a high-sampling rate (100 kHz) loop antenna installed at subauroral latitudes in Athabasca (ATH), Canada (54.7N, 246.7E, L=4) we have been able to continuously monitor VLF/ELF emissions since September 2012. Several types of VLF/ELF emissions were observed, including chorus, hiss and quasi-periodic emissions. We report statistics of VLF/ELF emissions using a one-year data set from November 1, 2012 until October 31, 2013. Using 10 minute and 24 hour spectra, we selected clearly defined emissions with a minimum intensity of 2.10^-5 pT²/Hz and recorded their starting time, duration, frequency range and spectral characteristics. This data set allowed us to calculate their occurrence rate as a function of AE, Dst and other geomagnetic parameters. We found similar occurrence rates on the ground in all cases, showing a peak around 07 MLT (7-10%) and a minimum from 18 to 02 MLT (1-3%), in agreement with previous satellite measurements at the geomagnetic equator. However, occurrence rates on the ground can be 8 times lower than those observed at the equator. This could be caused by the ionosphere preventing some frequencies to go all the way through, but could also suggest an interference in the propagation process between the generation region in the geomagnetic equator and the ground.

To investigate this, we compared this data set of VLF/ELF emissions with the observations made by the Van Allen Probes near the equatorial plane. We found 77 conjugate events for which the footprints of either the Van Allen Probes A or B (or both) were within 1000 km of ATH. Using the L2 magnetic field data from the EMFISIS instrument (CDF files available at https://emfisis.physics.uiowa.edu/), we were able to determine that the satellites observed VLF/ELF emissions for at least 54 of those events, suggesting that the spatial extent of the emissions is large. Within these events, we found 8 cases showing similar frequency and spectral features on the ground and on the satellite(s). We report the preliminary conclusions from our analysis of these events.