Quiet-time Solar Wind Superhalo Electrons

Superhalo electrons carry important information on the electron acceleration in the solar wind. Using the STEREO/STE electron measurements at ~2-20 keV and WIND/3DP measurements at ~20-200 keV, we find that solar wind superhalo electrons are present in the interplanetary medium (IPM) even in absence of any significant solar and interplanetary activity. The observed superhalo electrons generally have a nearly isotropic angular distribution and a power-law energy spectrum, J~E-β. The spectral index β ranges from ~1.5 to ~3.7, with an average of ~2.4. Both the superhalo power-law spectrum and anisotropy show no obvious correlation with sunspot number, solar flares, solar wind core population, etc. These superhalo electrons may form a quiet-time energetic electron background/reservoir in the IPM. They may originate from nonthermal processes related to the acceleration of solar wind, followed by scattering into isotropic angular distributions in the IPM. Another possibility is that superhalo electrons could be formed mainly due to acceleration by wave-particle interactions through the IPM.