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PITCH-ANGLE DISTRIBUTIONS AND TEMPORAL VARIATIONS OF 0.3–300 keV SOLAR IMPULSIVE ELECTRON EVENTS

We investigate the propagation of ∼0.3–300 keV electrons in five solar impulsive electron events, observed by the WIND three-dimensional Plasma and Energetic Particle instrument, that have rapid-rise and rapid-decay temporal profiles. In two events, the temporal profiles above 25 keV show a second peak of inward-traveling electrons tens of minutes after the first peak, followed by a third peak due to outward-traveling electrons minutes later—likely due to reflection/scattering first at ∼0.7–1.7 AU past the Earth, and then in the inner heliosphere inside 1 AU. In the five events, below a transition energy E0 (∼10–40 keV), the pitch-angle distributions are highly anisotropic with a pitch-angle width at half-maximum (PAHM) of <15° (unresolved) through the time of the peak; the ratio Λ of the peak flux of scattered (22.°5–90° relative to the outward direction) to field-aligned scatter-free (0°–22.°5) electrons is ≲0.1. Above E0, the PAHM at the flux peak increases with energy up to 85° at 300 keV, and Λ also increases with energy up to ∼0.8 at 300 keV. Thus, low-energy electrons propagated essentially scatter-free through the interplanetary medium, while high-energy electrons experienced pitch-angle scattering, with scattering strength increasing with energy. The transition energy E0 between the two populations is always such that the electron gyroradius (ρe) is approximately equal to the local thermal proton gyroradius (ρTp), suggesting that the higher energy electrons were scattered by resonance with turbulent fluctuations at scale ≳ρTp in the solar wind.