Long-term Determination of Energetic Electron Precipitation into the Atmosphere Using Subionospheric VLF Perturbations

Energetic electron precipitation (EEP) is an important loss mechanism in the dynamic radiation belts. Obtaining accurate precipitating flux measurements is necessary for the understanding, modelling, and analysis of the spatial and temporal belt dynamics, their impact on the atmosphere, and ultimately climate. In this study we analyse observations of subionospherically propagating very low frequency (VLF) radio waves to determine EEP flux magnitudes from the outer radiation belt through their influence on the lower ionosphere. We analyse data from a radio wave receiver located in Sodankyla, Finland (SGO), part of the Antarctic-Arctic Radiation-belt Dynamic Deposition VLF Atmospheric Research Konsortia (AARDDVARK), which observes VLF radio signals from the US high-power narrow-band communication transmitter with call sign NAA located in Culter, Maine. We use a near-continuous dataset from November 2004 until March 2013 to determine long time period EEP into the atmosphere along this path which spans 3-8 L, i.e., under where these outer radiation belt processes occur. We determine quiet day curves (QDC) over the entire long time period and use these to identify propagation disturbances caused by EEP. Modelling of LWPC radio wave propagation is used to estimate the electron fluxes precipitating into the atmosphere from the observed amplitude disturbances. Correlation is preformed with other EEP measurements, geomagnetic indices, and chorus wave intensity to examine the link between geomagnetic indices, plasma wave occurrence and EEP flux magnitudes. We find that using a dynamically varying energy spectral gradient for precipitating fluxes in the modelling gives improvements in the extracted EEP flux magnitudes compared to the fixed gradient used in Clilverd et al. [2010]. Our method performs well during the summer months when the day-lit ionosphere is the most stable. However our approach is unusable during the winter-time as it grossly over-exaggerates precipitating fluxes because of the higher variability in the received signal amplitudes. During the summer months only we have obtained 611 days worth of reasonable NAA-SGO fluxes over the 2004-2013 period. These fluxes agree well with POES BLC measurements during EEP events. Our method of EEP detection is also sensitive to measuring flux magnitudes below the noise floor of the POES instruments. A case study is also performed contrasting the NAA-SGO extracted EEP fluxes presented here to EEP measurements from a different AARDDVARK path recently published in Simon Wedlund et al. [2014]