SLAMS-Propelled Electron Acceleration at High-Mach Number Astrophysical Shocks

Short Large Amplitude Magnetic Structures (SLAMS) are frequently detected during spacecraft crossings over the Earth bow shock. We investigate the existence of such structures at astrophysical shocks, where they could result from the steepening of cosmic-ray (CR) driven waves. Using kinetic particle-in-cell simulations, we study the growth of SLAMS and the appearance of associated transient shocks in the upstream region of quasi-parallel, non-relativistic, high-Mach number collisionless shocks. We find that high-energy CRs significantly enhance the transverse magnetic field within SLAMS, producing highly inclined field lines. As SLAMS are advected towards the shock, these fields lines form an intermittent superluminal configuration which traps magnetized electrons at fast shocks. Due to their oscillatory nature, SLAMS are periodically separated by subluminal gaps with lower transverse magnetic field strength. In these regions, electrons diffuse and accelerate by bouncing between the shock and the approaching SLAMS region through a mechanism that we call quasi-periodic shock acceleration (QSA). We analytically derive the distribution of electrons accelerated via QSA, $f(p)\sim p^{[-4.7,-5.7]}$, which agrees well with the simulation spectra. We find that the electron power law remains steep until the end of our longest runs, providing a possible explanation for the steep electron spectra observed at least up to GeV energies in young and fast supernova remnants.