Shock Acceleration of 1-100 Kev Electrons at Earth`s Bow Shock II

We present a statistical study of shock acceleration of ~1-100 keV solar wind suprathermal eletrons at Earth's bow shock, by using Wind 3D plasma and energetic particle measurements in ambient solar wind and Magnetospheric Multiscale mission measurements in shock downstream. We select 74 shock cases from 2015 October to 2017 January and classify them into 4 types according to the spectral shape in downstream: no significant acceleration (type 0, 23 of all), power-law spectrum only at low energy (type 1, 24 of all), power-law spectrum with high energy tail (type 2, 16 of all) and double-power-law spectrum (type 3, 11 of all). We find that shock cases in type 3 have the largest flux enhancement J_dn/J_ab (~1000) and θ_Bn (~70 % of type 3 with θ_Bn>80°), which is the angle between shock normal and upstream magnet field, followed by type 2, type 1 and type 0. It indicates that the strongest shock electron acceleration can generate a double-power-law spectrum and more likely occurs at shocks with larger θ_Bn. Besides, J_dn/J_ab has a significant positive correlation with θ_Bn, and more than 50 % shocks show a filed-perpendicular pitch angle distributions of J_dn/J_ab in downstream, which support shock drift acceleration mechanism. We also find that the normalized drift time T_d/T_tr of suprathermal electrons at all energy channels shows a positive correlation with θ_Bn, which implicates that θ_Bn may be a key parameter affecting the drift process of suprathermal electrons in shock ramp.