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

Abstract:

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. Among 74 shock cases (1 quasi-parallel shock, 73 quasi-perpendicular shocks) during 2015 October - 2017 January, there are 23 (type 0) without significant electron acceleration after shock passage, 24 (type 1) with power-law-spectrum, J∝W^-β1, at ~1-3 keV in downstream, 16 (type 2) with power-law-spectrum at the same energy range and significant flux enhancement above 30 keV in downstream, and 11 (type3) with a clear double-power-law spectrum, J∝W^-β1 (J∝W^-β2) when W>>W_tr (W<<W_tr), bending down at W_tr~20-90 keV in downstream. The spectral indexes at ~1-3 keV, β₁, range from 2.5 to 5, while the spectral indexes above 30 keV, β₂, range from 5 to 9, and all the spectral indexes shows no correlation with those in ambient solar wind. To explain the formation of double-power-law spectrum in downstream of type 3 shocks, we use shock drift acceleration model and find that the drift time T_d of suprathermal electrons in shock ramp is close to the transmission time T_tr of bulk plasma traveling through shock ramp, T_d/T_tr~1-3 (~0.5-1) when W<W_tr (W>W_tr) , and the gyration scale of electrons at W_tr, D_g(W_tr), is close to the ramp thickness D_ramp. Besides, T_d/T_tr increases (decreases) with energy at W>>W_tr (W<<W_tr), which can lead to a flattened (steepened) energy spectrum in downstream. All of these make a quantitative explanation of the formation of double-power-law spectrum in shock downstream.