Bimodal Electron Fluxes of Nearly Relativistic Electrons during the Onset of Solar Particle Events: 1. Observations

TL;DR

This study analyzes nearly relativistic electron fluxes during solar energetic particle events, revealing bimodal pitch angle distributions at Wind spacecraft linked to reflection at Earth's bow shock, contrasting with uni-modal distributions at ACE.

Contribution

It provides new insights into the origin of bimodal electron distributions during solar events, highlighting the role of Earth's bow shock in electron reflection.

Findings

Wind observed bimodal pitch angle distributions of electrons.

ACE observed single-peaked pitch angle distributions.

Electron reflections are linked to Earth's bow shock or magnetosheath.

Abstract

We report for several solar energetic particle events 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 (PAD), 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 ~ 70 Earth radii away from 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 bi-modal electron PADs are due to reflection or scattering at an obstacle located at a distance of less than ~ 150 Earth radii in the anti-sunward direction, compatible with the bow shock or magnetosheath of the magnetosphere of the Earth.