Energetic outer radiation belt electron precipitation during recurrent solar activity

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 similar to 27 days apart, with daily Sigma Kp reaching similar to 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 300 keV and similar to 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 similar to 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.

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