Electron Injection into Fermi Acceleration in Quasiparallel Collisionless Shocks: Combining Hybrid Simulations with Test Particle Acceleration
July 22, 2019
In situ observations of energetic particles at the Earth's bow-shock have long created an opinion that electrons are most efficiently accelerated (injected) in a quasi-perpendicular, while protons in quasi-parallel shock geometry. Shocks that deemed responsible for the production of cosmic ray electrons and their radiation from sources such as supernova remnants are, however, much more powerful and larger. Their remote observations suggest that electrons are accelerated very efficiently in the quasi-parallel geometry. We investigate the possibility that protons accelerated to high energies create sufficient wave turbulence required for the electron injection into the diffusive shock acceleration. Numerically we do it by means of 1D hybrid simulations, in which we introduce electrons as test-particles, aiming to see how the "hybrid" fields act on the individual electrons. The reduced spatial dimensionality allowed us to dramatically increase the number of macro-ions per cell and achieve the converged results for the velocity distributions of electrons. Our simulations show, that the electron-to-proton temperature ratio is a decreasing function of the shock Mach number. Furthermore, a saturation of the temperature ratio at higher Mach numbers is noticeable. This behavior is in agreement with the electron-ion temperature ratio observed in Balmer-dominated shocks [P. Ghavamian et al., Space Science Reviews, 178, 2013].
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