Cosmic ray current-driven turbulent mean-field dynamo effect
by
Prof.Igor Rogachevskii(Ben-Gurion University of the Negev)
→
Europe/Stockholm
122:026
122:026
Description
We consider the possibility that a dynamo effect is initiated by cosmic ray instabilities such as the Bell instability that excite left-right asymmetric small-scale flows that ultimately lead to turbulence. This non-resonant instability is excited in a plasma with a given cosmic ray current and causes growing Alfven waves with a preferred circular polarization--depending on the relative orientations of cosmic ray current ambient magnetic field. We analyze three contributions to the alpha effect caused by: (i) non-zero kinetic helicity produced by the non-resonant Bell-instability; (ii) interaction of the mean cosmic ray current with the small-scale anisotropic incompressible part of the resulting turbulence; and (iii) interaction of the mean cosmic ray current with small-scale compressible part of the turbulence. We also perform direct numerical simulations (DNS) and use the test-field method to determine the alpha effect and the turbulent magnetic diffusivity. As follows from DNS, the dynamics of the instability has the following stages: (i) in the early stage, the small-scale Bell instability that results in the production of small-scale turbulence is excited; (ii) in the intermediate stage, there is formation of large-scale magnetic structures due to large-scale dynamo instability; and (iii) in the last stage, there is fully developed turbulence at large-scales. The results of DNS are in a good agreement with the theoretical predictions. The mean-field dynamo mechanism can be important in the generation of the observed large-scale magnetic fields in heliospheric shocks and large-scale magnetic field amplification associated with cosmic ray production and diffusive shock acceleration in supernova remnants (SNR) and blast waves from gamma ray bursts. Magnetic field amplification by Bell turbulence in SNR is found to be modest because of the limited time available to the expanding remnant. Limits on magnetic field growth in longer lived systems are derived. Application to unconfined intergalactic cosmic rays is also discussed. It is suggested that the resonant cosmic ray streaming Alfven instability, though not possibly to include in an MHD simulation, plays the same role in generating left-right asymmetric small-scale turbulence.
