Ninety degrees pitch angle enhancements of suprathermal electrons associated with interplanetary shocks

Abstract

AbstractWe report the results of the first systematic analysis of 90° pitch angle (PA) enhancements or the ring distributions of suprathermal (E ∼70 eV–2 keV) electrons at interplanetary (IP) shocks. We analyze 2 h time intervals around 232 IP shocks observed by the two STEREO spacecraft between 2007 and 2011. The ring distributions were detected downstream of 114 events (49%). In 52 (22.4%) cases they were detected at the shock ramp. We also found 90° enhancements upstream of 11 (4.7%) events. Statistical analysis of basic shock properties did not reveal substantial differences between the shocks that are associated with the enhancements and those that are not. The data from the STEREO/WAVES instruments revealed that the 90° PA enhancements tend to be associated with magnetic and electric field fluctuations. Although at this point we do not have a satisfactory explanation for the mechanism that produces these distributions, our findings suggest that wave‐particle interactions play a role, while pure focusing and mirroring effects due to adiabatic motion of electrons across the shock fronts cannot fully account for the observations.