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THE ANGULAR DISTRIBUTION OF SOLAR WIND SUPERHALO ELECTRONS AT QUIET TIMES

The angular distribution of superhalo electrons provides important information on the generation of superhalo electrons in the solar wind. Here we present a comprehensive study of the angular distribution of ∼20–200 keV superhalo electrons measured at 1 AU by the WIND 3DP instrument during quiet times from 1995 January through 2005 December. For a given energy, we define the anisotropy of superhalo electrons (with respect to the interplanetary magnetic field) as A = 2 ( J out − J in ) J out + J in , ?> where Jout (Jin) is the observed differential flux of electrons traveling outward from (inward toward) the Sun. We found that for ∼96% of the selected quiet-time samples, superhalo electrons have isotropic angular distributions, while for ∼3% (∼1%) of quiet-time samples, superhalo electrons have outward-anisotropic (inward-anisotropic) angular distributions. All three groups of superhalo electrons show no correlation with the local solar wind plasma, interplanetary magnetic field, turbulence, and energetic electrons accelerated/reflected at the terrestrial bow shock. For the generation of quiet-time superhalo electrons, one possibility is that they are accelerated by nonthermal processes related to the solar wind source, followed by strong scattering/reflection in the interplanetary medium. Another possibility is that these superhalo electrons are formed due to the acceleration of solar wind halo/strahl electrons, e.g., by turbulence, throughout the interplanetary medium.