The plasma filamentation instability in one dimension: nonlinear evolution
Abstract
The plasma filamentation instability or beamWeibel instability generates magnetic fields and accelerates particles in collisionless astrophysical plasma. This instability has been examined with multidimensional particleincell (PIC) simulations, demonstrating the formation of current flux tubes. Such simulations could not model a statistically significant number of filaments. Here, we model with a PIC simulation the filamentation instability that is driven by nonrelativistic, cool electron beams in one spatial dimension at an unprecedented resolution. We show unambiguously that the gradient of the magnetic pressure which develops during the quasilinear evolution of the filamentation instability, gives rise to an electrostatic field component. The interplay of the magnetic and electrostatic fields results in a wavenumber spectrum of the magnetic field that is a powerlaw, which has been reported previously for multidimensional PIC simulations. The magnetic field power spectrum decreases with the exponent 5.7 and that of the electrostatic field with 3.8, yielding a ratio of 3:2. The electromagnetic fields thermalize the electrons. The electrons develop a velocity distribution in the simulation direction that decreases exponentially at low speeds and faster at high speeds. The filamentation instability can thus not efficiently accelerate electrons to high energies. The filaments develop into a stationary final state. The probability distribution of the filament sizes is a Gumbel distribution. In astrophysical settings, this implies that the long exponential tail of this distribution may lead with a reasonable probability to large current filaments, if the filamentation instability develops in a large enough volume. The coherent magnetic fields of large filaments are required to explain the synchrotron emissions of gamma ray bursts.
 Publication:

New Journal of Physics
 Pub Date:
 August 2007
 DOI:
 10.1088/13672630/9/8/247
 Bibcode:
 2007NJPh....9..247R