An Abrupt Ending of Long Dormant Outer Radiation Belt Electrons: The External and Internal Conditions That Made This Possible

During the most recent solar minimum, the solar wind has been very quiet and the outer radiation belt electron flux level has been unusually low. This extraordinary period of extremely low-level electron flux ended abruptly on 14 February 2009, when the electron flux (>2 MeV) at geosynchronous orbit increased by more than two orders of magnitude. This increase occurred during passage of a ~600 km/s high-speed solar wind stream that was fronted by a strong stream interaction region and that produced a Dst storm of -40 nT. Two relatively low-speed (~350 km/s) interplanetary coronal mass ejections, which passed Earth earlier on 26 January and 4 February, produced Dst storms of -40 nT and -50nT, respectively, but did not produce any substantial enhancement in > 2 MeV electrons at geosynchronous orbit. The >2 MeV electron flux gradually decayed back to pre-existing levels over the 11 days after the 14 February enhancement. Interestingly, a high-speed solar wind stream, which peaked at ~ 675 km/s on 27 February, did not re-enhance the > 2 MeV electron flux at geosynchronous orbit significantly. Perhaps this latter high-speed stream was not fronted by a strong stream interaction region. Our analysis addresses the question of why only the 14 February wind stream out of this sequence of solar wind events produced a significant increase in the > 2 MeV electron flux in the outer radiation belt.