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Dispersion and thermal effects on electromagnetic instabilities in the precursor of relativistic shocks

Abstract : Fermi acceleration can develop efficiently at relativistic collisionless shock waves provided the upstream (unshocked) plasma is weakly magnetized. This has been both indicated by analytical theory and observed in numerical particle-in-cell simulations. At low magnetization, the large size of the shock precursor indeed provides enough time for electromagnetic micro-instabilities to grow, and such micro-instabilities generate small-scale turbulence that in turn provides the scattering required for particles to undergo Fermi cycles at superluminal relativistic shock waves. The present paper extends our previous analysis on the development of these micro-instabilities to account for the finite angular dispersion of the beam of reflected and accelerated particles and to account for the expected heating of the upstream electrons in the shock precursor. Indeed, we argue that the electrons can be significantly heated during their travel in the shock precursor and that they may even reach equipartition with protons, in agreement with recent numerical simulations. We show that the oblique two-stream instability may operate down to values of the shock Lorentz factor γsh∼ 10 (corresponding to a relatively large angular dispersion of the beam) as long as the electrons of the upstream plasma remain cold, while the filamentation instability is strongly inhibited in this limit; however, as electrons get heated to relativistic temperatures, the situation becomes opposite and the two-stream instability becomes inhibited while the filamentation mode becomes efficient, even at moderate values of the shock Lorentz factor. The peak wavelength of these instabilities migrates from the inertial electron scale towards the proton inertial scale as the background electrons get progressively heated during the crossing of the shock precursor. We also discuss the emergence and the role of current-driven instabilities upstream of the shock. In particular, we show that the returning and accelerated particles give rise to a transverse current through their rotation in the background magnetic field. We find that the compensating current in the background plasma can lead to a Buneman instability which provides an efficient source of electron heating.
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Martin Lemoine, Guy Pelletier. Dispersion and thermal effects on electromagnetic instabilities in the precursor of relativistic shocks. Monthly Notices of the Royal Astronomical Society, Oxford University Press (OUP): Policy P - Oxford Open Option A, 2011, 417, pp.1148-1161. ⟨10.1111/j.1365-2966.2011.19331.x⟩. ⟨insu-03645866⟩

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