Electron Acceleration in Non-relativistic Quasi-perpendicular Collisionless Shocks

Abstract

The origin of nonthermal emission observed from a variety of astrophysical objects is still a major unresolved issue in plasma astrophysics. Shocks at SNRs, with the help of a universal acceleration mechanism (i.e., diffusive shock acceleration; DSA), are widely believed to be the most probable acceleration sites of galactic cosmic rays (CRs). The underlying assumption of DSA is that only particles with Larmor radius much larger than the shock width can cross the shock and enter the acceleration process. This is especially challenging for thermal electrons due to their small Larmor radii. In non-relativistic quasi-perpendicular shocks without significant proton acceleration, whether electrons can be injected into DSA by self-driven upstream turbulence is not well-addressed in the literature. In this thesis, I try to answer this question by performing large scale particle-in-cell (PIC) simulations and magnetohydrodynamic-particle-in-cell (MHD-PIC) simulations. 1D PIC simulations show that electrons are injected into DSA through repeated cycles of shock drift acceleration (SDA) and the scattering of self-driven upstream waves. Multi-dimensional PIC simulations show different electron acceleration efficiencies with different background magnetic field geometries. 2D out-of-plane shocks are much more efficient in electron acceleration compared to in-plane shocks and the acceleration efficiency in 3D shocks lies in between 2D in-plane and out-of-plane shocks. I demonstrate that both the pre-acceleration at the shock leading edge and the corrugations at the shock ramp affect the electron acceleration efficiency. For the second half of my thesis, I apply MHD-PIC method to study electron acceleration in oblique shocks for larger transverse size and longer time scale. I develop a simple but more realistic electron injection prescription motivated by PIC simulations. The MHD-PIC simulations reproduce the most essential features of the shock structure and electron acceleration process. Quasi-perpendicular shocks can self-regulate how many particles they can take in response to different injection fractions by creating shock corrugations, making MHD-PIC model more robust for studying long term particle acceleration process without a detailed understanding of microphysics. By combing the results from both PIC simulations and MHD-PIC simulations, we can gain more insights into the physics of electron acceleration at different scales in astrophysical systems.