itch angle distributions and temporal variations of 0.3-300 keV solar impulsive electron events

We investigate pitch angle distributions (PADs) and temporal variations for five solar impulsive electron events with a rapid rise and rapid decay, observed by the WIND 3D Plasma & Energetic Particle instrument in the energy range from ∼0.3 to 300 keV. Below a transition energy E₀ ( ∼10-30 keV), the PADs are highly anisotropic with a pitch-angle width at half maximum (PAHM) of <15^o (unresolved) through the peak; the ratio of the peak flux of scattered (22.5-90^o) to field-aligned scatter-free (0-22.5^o, relative to the outward direction) electrons is <0.1. Above E₀, the PADs become broader and the PAHM at peak increases with energy up to 85^o at 300 keV; the peak-flux ratio of scattered to scatter-free electrons also increases with energy up to ∼0.8 at 300 keV. Thus, in these events, electrons with energies below E₀ propagated essentially scatter-free through the interplanetary medium, while electrons at higher energies experienced pitch-angle scattering, with scattering strength increasing with energy. In the five events studied, the transition energy E₀ between the two populations occurs where the electron gyroradius (ρ _e) is equal to the local thermal proton gyroradius (ρ _Tp), suggesting that the higher energy electrons were scattered by resonance with turbulent fluctuations at scale ≳ ρ _Tp in the solar wind. In two events, the temporal profiles above 25 keV also show a second peak of inward-traveling electrons tens of minutes after the first peak, possibly due to reflection/scattering at ∼0.5-1 AU past the Earth, and a third peak due to outward-traveling electrons minutes later, likely caused by reflection/scattering of those inward-traveling populations between the Sun and Earth.