Ground-based estimates of outer radiation belt energetic electron precipitation fluxes into the atmosphere

AARDDVARK data from a radio wave receiver in Sodankyla, Finland have been used to monitor transmissions across the auroral oval and just into the polar cap from the very low frequency communications transmitter, call sign NAA (24.0 kHz, 44 degrees N, 67 degrees W, L = 2.9), in Maine, USA, since 2004. The transmissions are influenced by outer radiation belt (L = 3-7) energetic electron precipitation. In this study, we have been able to show that the observed transmission amplitude variations can be used to determine routinely the flux of energetic electrons entering the upper atmosphere along the total path and between 30 and 90 km. Our analysis of the NAA observations shows that electron precipitation fluxes can vary by 3 orders of magnitude during geomagnetic storms. Typically when averaging over L = 3-7 we find that the >100 keV POES "trapped" fluxes peak at about 10(6) el. cm(-2) s(-1) sr(-1) during geomagnetic storms, with the DEMETER >100 keV drift loss cone showing peak fluxes of 105 el. cm(-2) s(-1) sr(-1), and both the POES >100 keV "loss" fluxes and the NAA ground-based >100 keV precipitation fluxes showing peaks of similar to 10(4) el. cm(-2) s(-1) sr(-1). During a geomagnetic storm in July 2005, there were systematic MLT variations in the fluxes observed: electron precipitation flux in the midnight sector (22-06 MLT) exceeded the fluxes from the morning side (0330-1130 MLT) and also from the afternoon sector (1130-1930 MLT). The analysis of NAA amplitude variability has the potential of providing a detailed, near real-time, picture of energetic electron precipitation fluxes from the outer radiation belts.