Magnetic field generation in a jet-sheath plasma via the kinetic Kelvin-Helmholtz instability

Abstract. We have investigated the generation of magnetic fields associated with velocity shear between an unmagnetized relativistic jet and an unmagnetized sheath plasma. We have examined the strong magnetic fields generated by kinetic shear (Kelvin–Helmholtz) instabilities. Compared to the previous studies using counter-streaming performed by Alves et al. (2012), the structure of the kinetic Kelvin–Helmholtz instability (KKHI) of our jet-sheath configuration is slightly different, even for the global evolution of the strong transverse magnetic field. In our simulations the major components of growing modes are the electric field E_z, perpendicular to the flow boundary, and the magnetic field B_y, transverse to the flow direction. After the B_y component is excited, an induced electric field E_x, parallel to the flow direction, becomes significant. However, other field components remain small. We find that the structure and growth rate of KKHI with mass ratios m_i/m_e = 1836 and m_i/m_e = 20 are similar. In our simulations saturation in the nonlinear stage is not as clear as in counter-streaming cases. The growth rate for a mildly-relativistic jet case (γ_j = 1.5) is larger than for a relativistic jet case (γ_j = 15).