Bow Shock Acceleration of Solar Wind Suprathermal electrons

We present a statistical study of the in-situ electron acceleration of solar wind suprathermal electrons at the Earth's bow shock, for 58 shock crossing events observed by MMS from September 2015 to March 2017. Among the 58 shock crossing events, 57 show quasi-perpendicular shock geometries, with significant enhancements of suprathermal electrons across the shock observed in 45 events, and only one has a quasi-parallel shock geometry that shows no obvious electron enhancement across the shock, after applying the Rankine-Hugoniot shock fitting technique. For these 45 events, the differential flux of shocked suprathermal electrons tend to have a double-power-law energy spectrum at ~1-100 keV, J～E^β , with an average break energy E₀ of ~ 45 keV, an average spectral index of β₁~3.5 at energies below E₀ and of β₂~7 at energies above. the low-energy spectral index β1 shows a negative correlation with θ_Bn and M_f, while the high-energy index β₂ has no correlation with the shock parameters. Furthermore, the pitch angle distribution of shocked electrons peaks near 90° at energies below ~ 10 keV in most event. Finally, we find that the gyration scale of electrons at E₀ is similar to the estimated thickness of shock ramp, likely leading to an efficient (inefficient) electron acceleration at energies below (above) E₀.