Bimodal fluxes of near-relativistic electrons during the onset of solar particle events

摘要

We report for several solar energetic particle events (SEPs) intensity and anisotropy measurements of energetic electrons in the energy range ～27 to ～500 keV as observed with the Wind and ACE spacecraft in June 2000. The observations onboard Wind show bimodal pitch angle distributions (PADs), whereas ACE shows PADs with one peak, as usually observed for impulsive injection of electrons at the Sun. During the time of observation Wind was located upstream of the Earth's bow shock, in the dawn-noon sector, at distances of ～40 to ～70R_Efrom the Earth, and magnetically well connected to the quasi-parallel bow shock, whereas ACE, located at the libration point L1, was not connected to the bow shock. The electron intensity-time profiles and energy spectra show that the backstreaming electrons observed at Wind are not of magnetospheric origin. The observations rather suggest that the bimodal electron PADs are due to reflection or scattering at an obstacle located at a distance of less than ～150R_E in the antisunward direction, compatible with the bow shock or magnetosheath of the magnetosphere of the Earth. For a modeling of the observations, we have performed transport simulations which include the effects of pitch angle diffusion, adiabatic focusing, and reflection at a boundary close to the point of observation. The results of the simulations demonstrate that the bimodal PADs are compatible with the reflection of electrons at a nearby boundary, at distances of ～70R_E. This finding is supported by the orbital configuration and the magnetic field direction: Whereas ACE is not connected, Wind is well connected to the magnetosphere of the Earth.