Shock Acceleration of ~1-100 Kev Electrons at Earth's Bow Shock

We present a statistical study of in-situ shock acceleration of ~1-100 keV solar wind suprathermal electrons at Earth’s bow shock, by using Wind 3D plasma and energetic particle measurements in ambient solar wind and MMS measurements in shock downstream. We pick out 74 shock cases (1 quasi-parallel shock, 73 quasi-perpendicular shocks) during 2015 October - 2017 January, and classify them into 4 types according to their energy spectra in downstream: type 0 (23 cases) without significant electron acceleration after shock passage, type 1 (24 cases) with power-law spectrum, J ∝ε^β1_dn, at ~0.8-10 keV, type 2 (16 cases) with power-law-spectrum at ~0.8-10 keV and significant flux enhancement above 30 keV, and type 3 (11 cases) with a clear double-power-law spectrum, J ∝ ε^β1_dn (J ∝ ε^β2_dn) when ε « ε^dn_tr  (ε » ε^dn_tr), bending down at ε^dn_tr ~20-90 keV. The spectral indexes at lower energies for type 1, type 2 and type 3, β₁^dn, range from 2.5 to 5, while the spectral indexes at higher energies for type 3, β₂^dn, range from 4 to 9, and all the spectral indexes have no significant correlation with those in ambient solar wind. Among the 4 types, type 3 is the strongest acceleration with the largest flux enhancement and the lowest β₁^dn. Besides, we find that the flux ratio between downstream and ambient solar wind J_dn/J_ab is field-perpendicular for most cases in both low and high energies, and J_dn/J_ab (β₁^dn) has positive (negative) correlations with θ_Bn and magnetic field compression ratio, r_B, which favor the shock drift acceleration (SDA) mechanism. However, J_dn/J_ab has no correlation with the drift electric field E_d, while the normalized drift time, T_d/T_tr, has a positive correlation with θ_Bn, it suggests that θ_Bn can influence electron drift time and thus influence the acceleration efficiency.