Particle-in-cell simulation of a mildly relativistic collision of an electron-ion plasma carrying a quasi-parallel magnetic field: Electron acceleration and magnetic field amplification at supernova shocks

摘要

Plasma processes close to SNR shocks result in the amplification of magneticfields and in the acceleration of electrons, injecting them into the diffusiveacceleration mechanism. The acceleration of electrons and the B fieldamplification by the collision of two plasma clouds, each consisting ofelectrons and ions, at a speed of 0.5c is investigated. A quasi-parallelguiding magnetic field, a cloud density ratio of 10 and a plasma temperature of25 keV are considered. A quasi-planar shock forms at the front of the denseplasma cloud. It is mediated by a circularly left-hand polarizedelectromagnetic wave with an electric field component along the guidingmagnetic field. Its propagation direction is close to that of the guiding fieldand orthogonal to the collision boundary. It has a low frequency and awavelength that equals several times the ion inertial length, which would beindicative of a dispersive Alfven wave close to the ion cyclotron resonancefrequency of the left-handed mode (ion whistler), provided that the frequencyis appropriate. However, it moves with the super-alfvenic plasma collisionspeed, suggesting that it is an Alfven precursor or a nonlinear MHD wave suchas a Short Large-Amplitude Magnetic Structure (SLAMS). The growth of themagnetic amplitude of this wave to values well in excess of those of thequasi-parallel guiding field and of the filamentation modes results in aquasi-perpendicular shock. We present evidence for the instability of this modeto a four wave interaction. The waves developing upstream of the dense cloudgive rise to electron acceleration ahead of the collision boundary. Energyequipartition between the ions and the electrons is established at the shockand the electrons are accelerated to relativistic speeds.