Solar wind Suprathermal Electrons around the Corotating Interaction Regions at 1 AU

Here we present a statistical study of the 0.1-1 keV suprathermal electrons in the undisturbed and compressed slow/fast solar wind, around the 71 corotating interaction regions (CIRs) with good measurements from the WIND 3DP and MFI instruments from 1995 through 1997. For each of these CIRs, we select four 3-hour intervals to average the electron measurements in the undisturbed/compressed slow and fast wind. Then we separate the strahl and halo electrons according to their different behaviors in pitch angle distributions in the undisturbed and compressed solar wind. We fit both the strahl and halo energy spectra to a kappa function with an index κ and effective temperature T_eff, and calculate the pitch-angle width at half-maximum (PAHM) of the strahl population . For both the strahl and halo populations in the undisturbed/compressed slow and fast wind, we find that the fitted κ index strongly correlates with T_eff, similar to the quiet-time solar wind measurements (Tao et al., ApJ, 2016), while the integrated number density of both the strahl and halo over 0.1-1 keV shows a strong positive correlation with the electron core temperature T_e. The strahl number density also correlates with the interplanetary magnetic field strength and the strahl width is anti-correlated with the solar wind speed v_sw, in both slow and fast wind. These results suggest that corona temperature that determines the strahl number density could be correlated with the electron core temperature in the solar wind source region for both slow and fast wind, and the strahl number density may depend on the expansion rate of magnetic flux in the interplanetary medium. On the other hand, the density ratio of the halo over the core is relatively smaller in compressed solar wind than in undisturbed wind, while the ratio of the strahl over the core remains almost constant. For both the strahl and halo, the κ index shows no obvious difference between the undisturbed and compressed wind and between the slow and fast wind. These results suggest that CIRs at 1 AU do not contribute to the formation of halo electrons, and the halo can be converted to lower or higher energy electrons in CIRs.