The Angular Distribution of Quiet-time ~20-300 keV Superhalo Electrons in the Solar Wind

Abstract:

The angular distribution of solar wind superhalo electrons carries important information on the electron acceleration location and scattering in the interplanetary medium. Here we present a comprehensive study of the angular distribution of ~20–300 keV superhalo electrons measured at 1 AU by the WIND 3DP instrument during quiet-time periods from 1995 January through 2013 December. For quiet-time intervals, we re-bin the observed electron pitch angle distributions into the outward-traveling and inward-traveling bins, according the direction of interplanetary magnetic field (IMF). The inward-outward anisotropy of superhalo electrons at energy E is defined as A = 2(f_out – f_in)/(f_out + f_in), where f_out (f_in) is the average flux of outward-traveling (inward-traveling) electrons. We find that among all the ~640 quiet-time intervals, ~5% have an A > 0.1 (referred to as “outward events”), ~5% have an A < -0.1 (referred to as “inward events”), and ~90% have an |A| ≤ 0.1 (referred to as “isotropic events”). Isotropic events show no clear correlation with solar wind parameters (n_SW, V_sw and T_p), IMF and solar wind turbulence spectrum. Inward and outward events also have no association with the IMF and n_SW. But the occurrence ratio of outward (inward) events over all the events, α, roughly decreases (increases) with increasing V_SW. Moreover, for outward (inward) events, α roughly increases with ρ_e/ρ_Tp, where ρ_Tp is the solar wind thermal proton gyroradius that is related to the separation between the turbulence inertial and dissipation ranges. These results suggest that quite-time superhalo electrons are generally isotropic due to the wave-particle interaction in the interplanetary medium; outward-traveling (inward-traveling) superhalo electrons may come from the acceleration occurring beyond (within) 1 AU, probably by CIRs or turbulence. We will also present a case study of several quiet-time electron events with the anisotropy A increasing with the electron energy E.