Shock Acceleration via Idealized Short Large-amplitude Magnetic Structures

To accelerate cosmic rays (CRs) to ∼PeV energies, diffusive shock acceleration (DSA) in Galactic supernova remnants requires amplified magnetic turbulence in the shock to facilitate broadband Bohm diffusion of CRs. Recent particle-in-cell (PIC) shock simulations suggest that diffusion may be driven by short large-amplitude magnetic structures (SLAMS). Persistent SLAMS develop in the upstream of shocks with an Alfvénic Mach number M_A ≳ 20 via the Bell instability and nonlinear amplification/steepening. We employ test-particle simulations to examine particle transport and acceleration in model shocks populated by analytically prescribed, idealized SLAMS that approximate the magnetic structures of PIC shock simulations. We demonstrate that SLAMS drive broadband Bohm diffusion of magnetized electrons (i.e., electron Larmor radius r_L ≪ magnetic-structure length λ_AM) and unmagnetized electrons (i.e., r_L ≫ λ_AM). We show that SLAMS accelerate unmagnetized electrons via DSA, and magnetized electrons via shock drift acceleration (SDA) and quasi-periodic shock acceleration (QSA). QSA, a novel Fermi-type mechanism, proceeds via rapid scattering in the converging flow between the first upstream magnetic structure and the near-downstream, shock-amplified field. QSA becomes vital for maintaining electron acceleration in shocks with superluminal SLAMS, which strongly advect magnetized electrons due to high inclination between field lines and the flow. We find that test-electron spectra are consistent with spectra from PIC simulations, indicating that the test-particle method can be used to extrapolate PIC results to longer spatial and temporal scales. Overall, our results suggest that SLAMS in M_A ≳ 20 shocks may play an important role in facilitating shock acceleration.