Quasi-parallel collisionless shock (re)formation and particle acceleration by (non)resonant micro-instabilities

Abstract

We present a theoretical model and simulations of quasi-parallel magnetized collisionless shock (re)formation and consequential particle acceleration. The two counter-streaming plasmas of comparable densities can excite the resonant instability. By using Particle-In-Cell simulations, we analyze the role of this strong-beam instability in shock triggering and subsequent formation of the initial return current. Consequently, this current excites a weak-beam instability ahead of the forming shock, which quickly becomes non-linear and starts to reform the shock. We find that ions reflected from the reforming shock barrier, begin to bounce between the approaching upstream instability and its part that is compressed and slowed down at the shock. Ions are thus accelerated in successive resonant reflections. Electrons experience the same acceleration mechanism, however, their reflections are due to the conservation of the magnetic moment. This DSA-like mechanism can contribute not only to the formation of supra-thermal part of the particle spectra, but it can also build up the non-thermal tail.