34 research outputs found
Saturation mechanism of the Weibel instability in weakly magnetized plasmas
The saturation mechanism of the Weibel instability is investigated
theoretically by considering the evolution of currents in numerous cylindrical
beams that are generated in the initial stage of the instability. Based on a
physical model of the beams, it is shown that the magnetic field strength
attains a maximum value when the currents in the beams evolve into the Alfven
current and that there exist two saturation regimes. The theoretical prediction
of the magnetic field strength at saturation is in good agreement with the
results of two-dimensional particle-in-cell simulations for a wide range of
initial anisotropy.Comment: 13 pages, 3 figures, REVTeX. Accepted for publication in Phys.
Plasma
A Possible Origin of Magnetic Fields in Galaxies and Clusters: Strong Magnetic fields at z~10?
We propose that strong magnetic fields should be generated at shock waves
associated with formation of galaxies or clusters of galaxies by the Weibel
instability, an instability in collisionless plasmas. The strength of the
magnetic fields generated through this mechanism is close to the order of those
observed in galaxies or clusters of galaxies at present. If the generated
fields do not decay rapidly, this indicates that strong amplification of
magnetic fields after formation of galaxies or clusters of galaxies is not
required. This mechanism could have worked even at a redshift of ~10, and
therefore the generated magnetic fields may have affected the formation of
stars in protogalaxies. This model will partially be confirmed by future
observations of nearby clusters of galaxies. Mechanisms that preserve the
magnetic fields for a long time without considerable decay are discussed.Comment: Accepted for publication in MNRA
Nonrelativistic collisionless shocks in weakly magnetized electron--ion plasmas: two-dimensional particle-in-cell simulation of perpendicular shock
A two-dimensional particle-in-cell simulation is performed to investigate
weakly magnetized perpendicular shocks with a magnetization parameter of 6 x
10^-5, which is equivalent to a high Alfv\'en Mach number M_A of ~130. It is
shown that current filaments form in the foot region of the shock due to the
ion-beam--Weibel instability (or the ion filamentation instability) and that
they generate a strong magnetic field there. In the downstream region, these
current filaments also generate a tangled magnetic field that is typically 15
times stronger than the upstream magnetic field. The thermal energies of
electrons and ions in the downstream region are not in equipartition and their
temperature ratio is T_e / T_i ~ 0.3 - 0.4. Efficient electron acceleration was
not observed in our simulation, although a fraction of the ions are accelerated
slightly on reflection at the shock. The simulation results agree very well
with the Rankine-Hugoniot relations. It is also shown that electrons and ions
are heated in the foot region by the Buneman instability (for electrons) and
the ion-acoustic instability (for both electrons and ions). However, the growth
rate of the Buneman instability is significantly reduced due to the relatively
high temperature of the reflected ions. For the same reason, ion-ion streaming
instability does not grow in the foot region.Comment: 24 pages, 23 figures, accepted for publication in Ap