Solar Wind ~20-300 keV Superhalo Electrons

Abstract:

High-energy superhalo electrons are present in the interplanetary medium even in absence of any solar activity, carrying important information on the electron acceleration in the solar wind. We present a statistical survey 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. The velocity distribution function of the observed quiet-time superhalo electrons generally fits to a power-law spectrum, f ~ v^-γ, with γ ranging from ~4 to ~10. The integrated density of these superhalo electrons at 20–300 keV, n_sup, ranges from 10⁻⁹ cm⁻³ to 10⁻⁵ cm⁻³. Both log(n_sup) and γ show a good correlation with the sunspot number, with larger density and softer spectrum (γ~ 6-8) at solar maximum, and smaller density and harder spectrum (γ~ 4-5) at solar minimum. The observed power-law spectrum also has no clear association with flares, CMEs, active regions and solar wind core populations, while it shows a weak (~0.3) correlation with in situ solar wind turbulence spectrum. These results suggest that the seed particles of quiet-time superhalo electrons could originate from the Sun, and their acceleration could mainly occur in the interplanetary medium, probably by the electron interaction with solar wind turbulence, or by acceleration at the CIRs.