Quiet-time Solar Wind Suprathermal Electrons of Different Solar Origins

Abstract:

The energy spectrum of solar wind strahl, halo, and superhalo electrons likely carries crucial information on their possible origin and acceleration at the Sun. Here we statistically investigate the energy spectrum of solar wind strahl/halo electrons at ∼0.1– 1.5 keV and superhalo electrons at ∼ 20– 200 keV measured by Wind/ 3D Plasma and Energetic Particle during quiet times from 1998 to 2014, according to the types of their Potential Field Source Surface– mapped coronal source regions ( CSRs) . We adopt the classification scheme developed by Zhao et al. to categorize the CSRs into four types: active region (AR) , quiet Sun (QS) , coronal hole (CH) , and helmet-streamer associated region (HS) . We find that for the quiet-time strahl, the AR and HS (QS and CH) correspond to a smaller (larger) kappa index κ_strahl with the most frequent value of 7– 8.5 (8.5– 10) and a larger (smaller) n_strahl with the most frequent value of 0.013– 0.026 cm⁻³ ( 0.006– 0.0013 cm⁻³) . For the quiet-time halo, κ_halo behaves similarly to κ_strahl , but n_halo appears similar among the four CSR types. For the superhalo, the AR (QS) corresponds to a larger (smaller) power-law index β with the most frequent value of 2.2– 2.4 (1.8– 2.0) , while the HS and CH have a β not different from either the AR or QS; n_sup appears similar, with the most frequent value of 3x10⁻⁸– 3x10⁻⁷ cm⁻³ , among the four CSR types. These results suggest that the strahl (superhalo) from the hotter CSRs tends to be more (less) efficiently accelerated.