A study of interplanetary propagation of solar impulsive ~1-100keV electron events

We study solar impulsive ~1-100keV electron events that exhibit clear velocity dispersion, as observed by the 3D Plasma and Energetic Particle (3DP) instrument on the WIND spacecraft. At the onset of those impulsive electron events, the outward travelling electrons are observed first, then the backward travelling electrons are detected, mostly in high energy range (>20keV) and sometimes in low energy range (<20keV). The backward population must be produced by scattering and/or magnetic mirroring in the interplanetary medium beyond 1 AU. For one impulsive event observed on December 5,1997, we determine the delay between the onset of the outward travelling electrons and the onset of the backward travelling electrons in different pitch angle ranges. We find that the delays for 27, 40 and 66 keV electrons, respectively, are 10, 5.8 and 7.3 minutes at ~10° pitch angle; 1.7,1.8 and 3 minutes at ~30° pitch angle; 10.5, 6 and 7.2 minutes at ~55° pitch angle; 6.2, 3.2 and 4.2 minutes at ~80° pitch angle.Those time differences are not inversly proportional to parallel velocity along the magnetic field B, so the simple magnetic mirror is unlikely to be an explanation. We investigate pitch angle scattering by magnetic field fluctuations as a possible mechanism.