E. P. Alves, T. Grismayer, S. F. Martins, F. Fiúza, R. A. Fonseca, L. O. Silva
Collisionless plasma instabilities are fundamental in magnetic field
generation in astrophysical scenarios, but their role has been addressed in
scenarios where velocity shear is absent. In this work we show that velocity
shears must be considered when studying realistic astrophysical scenarios,
since these trigger the collisionless Kelvin-Helmholtz instability (KHI). We
present the first self-consistent three-dimensional (3D) particle-in-cell (PIC)
simulations of the KHI in conditions relevant for unmagnetized relativistic
outflows with velocity shear, such as active galactic nuclei (AGN) and
gamma-ray bursts (GRBs). We show the generation of a strong large-scale DC
magnetic field, which extends over the entire shear-surface, reaching
thicknesses of a few tens of electron skin depths, and persisting on
time-scales much longer than the electron time scale. This DC magnetic field is
not captured by MHD models since it arises from intrinsically kinetic effects.
Our results indicate that the KHI can generate intense magnetic fields yielding
equipartition values up to \epsilon_B/\epsilon_p ~ 10^-3 in the electron
time-scale. The KHI-induced magnetic fields have a characteristic structure
that will lead to a distinct radiation signature, and can seed the turbulent
dynamo amplification process. The dynamics of the KHI are relevant for
non-thermal radiation modeling and can also have a strong impact on the
formation of relativistic shocks in presence of velocity shears.
View original:
http://arxiv.org/abs/1107.6037
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