Electron surfing acceleration in oblique magnetic fields

Abstract

Initially, inhomogeneous plasma jets, ejected by active galactic nuclei and associated with gamma-ray bursts, are thermalized by the formation of internal shocks. Jet subpopulations can hereby collide at Lorentz factors of a few. As the resulting relativistic shock expands into the upstream plasma, a significant fraction of the upstream ions is reflected. These ions, together with downstream ions that leak through the shock, form relativistic beams of ions that outrun the shock. The thermalization of these beams via the two-stream instability is thought to contribute significantly to plasma heating and particle acceleration by the shock. Here, the capability of a two-stream instability to generate relativistic field-aligned and cross-field electron flow, is examined for a magnetized plasma by means of a particle-in-cell (PIC) simulation. The electrons interact with the developing quasi-electrostatic waves and oblique magnetic fields. The simulation results bring forward evidence that such waves, by their non-linear interactions with the plasma, produce a highly relativistic field-aligned electron flow and electron energies, which could contribute to the radio synchrotron emissions from astrophysical jets, to ultrarelativistic leptonic subpopulations propagating with the jet and to the halo particles surrounding the accretion disc of the black hole

Formation of electrostatic structures by wakefield acceleration in ultrarelativistic plasma flows: Electron acceleration to cosmic ray energies

“…The proton beam is unstable against the two-stream instability, which we have previously considered for the same initial conditions 29 and we briefly summarize the results here. The two-stream instability leads to the growth of elec- FIG.…”

“…Indeed, previous simulations have shown that the two-stream instability cannot extract significant energy from the proton beam. 29 This implies, in turn, that the shockreflected proton beam may act as a battery that accumulates energy during one shock cycle.…”

“…Plasmas 13, 062905 ͑2006͒ trostatic waves at the wave number k u Ϸ p,e / c. The electric field potential eventually becomes strong enough to trap the electrons, and the trapped electrons are transported across B, accelerating them to Lorentz factors ⌫ ϳ m p / m e by a mechanism that is similar to the electron surfing acceleration. 20,21 The simulation in our previous work 29 was stopped at T s = 1270T p , at which a secondary instability started to develop.…”

“…By giving this second proton beam a lower temperature, it does not significantly participate in the plasma dynamics. 29 We do not further consider this second proton beam and, unless stated otherwise, the term proton beam will refer to the shock reflected proton beam. These initial conditions imply an excess of mobile positive charges ͑protons͒ over negative charges ͑electrons͒, which are compensated by the introduction of immobile negative charges, e.g., negatively charged dust impurities.…”

“…Initially, only the electrons are accelerated. These electrons can be accelerated to high energies by the relativistic electrostatic waves, [28][29][30][31] in particular if a weak magnetic field is present. As long as only the electrons are accelerated, the proton beam experiences negligible energy loss.…”