The Efficiency of Electron Acceleration by ICME-driven Shocks

We present a study of the efficiency of the acceleration of suprathermal electrons at collisionless shock waves driven by interplanetary coronal mass ejections (ICMEs), with the data analysis from both the spacecraft observations and test-particle simulations. The observations are from the 3DP/EESA instrument on board Wind during the 74 shock events listed in Yang et al., and the test-particle simulations are carried out through 315 cases with different shock parameters. A total of seven energy channels ranging from 0.428–4.161 keV are selected. In the simulations, using a backward-in-time method, we calculate the average downstream flux in the 90° pitch angle. On the other hand, the average downstream and upstream fluxes in the 90° pitch angle can also be directly obtained from the 74 observational shock events. In addition, the variation in the event number ratio with the downstream to upstream flux ratio above a threshold value in terms of the shock angle (the angle between the shock normal and upstream magnetic field), upstream Alfvén Mach number, and shock compression ratio is statistically obtained. It is shown from both the observations and simulations that a large shock angle, upstream Alfvén Mach number, and shock compression ratio can enhance the efficiency of the shock acceleration. Our results suggest that shock drift acceleration is more efficient in the electron acceleration by ICME-driven shocks, which confirms the findings of Yang et al.