Solar Impulsive Electron Events with Unusual Velocity Dispersions

A statistical survey of 26 solar impulsive electron events in the ∼1 to 300 keV range is presented, as observed by the 3D Plasma and Energetic Particle experiment on the Wind spacecraft. This study was triggered by results of ACE/EPAM observations (Roelof et al.) reporting the absence of velocity dispersion in solar energetic electrons events in the energy range of 40-300 keV. Earlier studies also reported the absence of dispersion in some events, whereas Roelof et al. claim to see no velocity dispersion in most of the events. The presented survey in this paper shows that besides normal dispersive events (14 out of 26), there are indeed events with unusual velocity dispersion (12 out of 26) showing nearly simultaneous onsets at 1 AU above 100 keV. A strong correlation between the peak flux and the absence of velocity dispersion is found: Events with high peak flux (≥0.2 cm-2 s-1 ster-1 eV-1 at 27 keV) show unusual velocity dispersion. This explains that earlier WIND/3DP surveys taken during solar minimum where events were smaller did not show events with unusual velocity dispersion.