90^◦ Pitch Angle Enhancements of Suprathermal Electrons Associated with the Interplanetary Shock on 24 April 2008

Using test particle simulations, the 90^◦ pitch angle enhancements of suprathermal electrons associated with the interplanetary shock on 24 April 2008 observed by STEREO-A spacecraft have been studied. Firstly, the initial distribution function for each pitch angle channel is obtained by fitting the observed distribution at a given time before the shock arrival, and then the time-backward method is used to calculate the pitch angle distributions of suprathermal electrons downstream of the shock for a given energy channel. Due to the higher resonance frequency of suprathermal electrons, the turbulence spectrum includes the dissipation range in which low-energy electrons resonate. Three energy channels with central energies of 215.76 eV, 151.67 eV, and 106.63 eV are simulated. The results show that a peak of the pitch angle distributions downstream of the shock obtained for these energy channels appears near the 90^◦ pitch angle. The enhancement at the 90^◦ pitch angle is in good agreement with the observations. The resonance between suprathermal electrons and the turbulence dissipation range in the process of shock acceleration is suggested to play a crucial role in this phenomenon. A thorough study of the electric and magnetic field fluctuations at interplanetary shocks can provide a better understanding of the nature of 90^◦ pitch angle enhancements of suprathemal electrons.