Energetic outer radiation belt electron precipitation during recurrent solar activity

Abstract

Transmissions from three U.S. VLF (very low frequency) transmitters were received at Churchill, Canada, during an event study in May to November, 2007. This period spans four cycles of recurrent geomagnetic activity spaced ∼27 days apart, with daily ΣKp reaching ∼30 at the peaks of the disturbances. The difference in the amplitude of the signals received during the day and during the night varied systematically with geomagnetic activity, and was used here as a proxy for ionization changes caused by energetic electron precipitation. For the most intense of the recurrent geomagnetic storms there was evidence of electron precipitation from 3 < L < 7 for 10–15 days after the peak of the disturbance, as measured by ΣKp and Dst. This was consistent with the time variation of the fluxes of Polar Operational Environmental Satellites (POES) >300 keV and ∼1 MeV trapped electrons, and also consistent with the daily average ULF (ultralow frequency) Pc1–2 power (L = 3.9) from Lucky Lake, Canada, which was elevated during the ∼1 MeV electron precipitation period. This suggests that Pc1–2 waves may play a role in outer radiation belt loss processes during this interval. We show that the >300 keV trapped electron flux from POES is a reasonable proxy for electron precipitation during recurrent high‐speed solar wind streams, although it did not describe all of the variability that occurred. While energetic electron precipitation can be described through a proxy such as Kp or Dst, careful incorporation of time delays for different electron energies must be included. Dst was found to be the most accurate proxy for electron precipitation during the weak recurrent‐activity period studied.