Statistical Study of Solar Impulsive Electron Events

This study with the WIND 3DP instrument of 380 solar impulsive electron events shows many different temporal delays with respect to the correlated radio type III bursts. The majority of events have a delayed electron injection time with respect to the radio type III bursts of about 14 minutes. This is similar to other published findings, but with more events and better statistics. To try and determine why some events showed on-time arrivals of electrons while most were delayed we analyzed their peak flux spectra and the frequency distribution of events. The frequency distribution for all events is a power law with a slope of 1.35+/-.05 for energies 30 keV to 300 keV. The peak flux spectra for individual events usually shows a double power law dependence with a steepening at 50 keV. The two slopes have average indices of -3.5 and -2, and are slightly correlated. However there was no difference in these distributions between on-time events and delayed events. It is also found that there is a strong tendency for events with in-situ detected Langmuir waves at 1 AU to be delayed less (12 minutes on average) than those without visible waves (17 minutes on average). An interesting finding is that electron events occurring during a radio type II burst show significantly harder spectra, indicating that shocks with type II bursts might be more efficient at accelerating higher energy particles.