Long‐term determination of energetic electron precipitation into the atmosphere from AARDDVARK subionospheric VLF observations

• Published in: Journal of geophysical research. Space physics 2015-03, Vol.120 (3), p.2194-2211
• Main Authors: Neal, Jason J., Rodger, Craig J., Clilverd, Mark A., Thomson, Neil R., Raita, Tero, Ulich, Thomas
• Format: Article
• Language: English
• Subjects: AARDDVARK network; Atmosphere; Atmospheric research; Disturbances; Electron precipitation; Electrons; Fluctuations; Flux; Geophysics; Ionosphere; Precipitation; radiation belts; Radio waves; Satellites; subionospheric VLF propagation; Summer; VLF; Wave propagation
• ISSN: 2169-9380; 2169-9402
• DOI: 10.1002/2014JA020689

We analyze observations of subionospherically propagating very low frequency (VLF) radio waves to determine outer radiation belt energetic electron precipitation (EEP) flux magnitudes. The radio wave receiver in Sodankylä, Finland (Sodankylä Geophysical Observatory) observes signals from the transmitter with call sign NAA (Cutler, Maine). The receiver is part of the Antarctic‐Arctic Radiation‐belt Dynamic Deposition VLF Atmospheric Research Konsortia (AARDDVARK). We use a near‐continuous data set spanning November 2004 until December 2013 to determine the long time period EEP variations. We determine quiet day curves over the entire period and use these to identify propagation disturbances caused by EEP. Long Wave Propagation Code radio wave propagation modeling is used to estimate the precipitating electron flux magnitudes from the observed amplitude disturbances, allowing for solar cycle changes in the ambient D region and dynamic variations in the EEP energy spectra. Our method performs well during the summer months when the daylit ionosphere is most stable but fails during the winter. From the summer observations, we have obtained 693 days worth of hourly EEP flux magnitudes over the 2004–2013 period. These AARDDVARK‐based fluxes agree well with independent satellite precipitation measurements during high‐intensity events. However, our method of EEP detection is 10–50 times more sensitive to low flux levels than the satellite measurements. Our EEP variations also show good agreement with the variation in lower band chorus wave powers, providing some confidence that chorus is the primary driver for the outer belt precipitation we are monitoring. Key Points Subionospheric VLF used to detect energetic electron precipitation Magnitudes agree with satellite observations but with higher sensitivity Variation agrees well with that of lower band chorus