Acceleration of Solar Wind Suprathermal Electrons at the Earth’s Bow Shock

Abstract

We present a statistical study of the in situ acceleration of solar wind suprathermal electrons at the Earth’s bow shock, utilizing the Wind measurements in ambient solar wind and MMS1 measurements around the bow shock from 2015 September to 2017 December. All the selected 74 cases show significant suprathermal electron acceleration at the bow shock. The observed power-law indexes of accelerated electron energy spectra are significantly larger than the first-order Fermi acceleration prediction and the flux enhancement ratio peaks near a 90° pitch angle, suggesting that the shock drift acceleration (SDA) process plays a crucial role in accelerating suprathermal electrons at the bow shock. According to the observed electron spectral characteristics, the 74 cases can be classified into Types 1, 2, 3 and 4. The electron acceleration efficiency roughly increases from Type 1 to Type 4. For the Type 4 cases with strong electron acceleration, the shocked suprathermal electrons show a double-power-law energy spectrum bending downwards at a break near ∼65 keV with a low-energy spectral index of ∼3.1 and high-energy index of ∼7.6. The observed break energy is comparable to a critical electron energy with its cross-shock gyrodiameter equal to the shock’s ramp thickness D ramp. At energies below (above) , the accelerated electrons can be effectively confined into (can easily escape from) the SDA process with a roughly energy-independent (energy-decreasing) drift time and a probably conserved (nonadiabatically reduced) magnetic moment, since their gyrodiameter is less (greater) than D ramp. Therefore, such an SDA process could produce the observed double-power-law spectrum bending downwards at a break energy that is associated with D ramp.