14-08-2013 13:30 hs.
Numerical studies of diffusion and amplification of magnetic fields in turbulent astrophysical plasmas
Dr Reinaldo Santos Lima – Universidad de San Pablo
Magnetic fields and turbulence are present in astrophysical flows of every scale: stellar interiors and atmospheres, molecular clouds, the warm and the hot phase of the interstellar medium (ISM) of the Milk Way, the ISM of external galaxies, the intracluster medium of galaxies (ICM). In general, these two ubiquitous phenomena are intrinsically related. During this talk, I will discuss two topics which illustrate this point. The first is related to star-formation in molecular clouds, where a major puzzle is how to transport magnetic fields in highly conducting fluids in the presence of turbulence. The other topic is related to the origin of the magnetic fields in the diffuse turbulent ICM which involves collisionless plasma effects.
Regarding the first topic, the requirement of magnetic flux diffusion during the gravitational collapse of molecular clouds in order to explain the observed magnetic field intensities in protostars (“magnetic flux problem”), and the formation of rotationally sustained protostellar discs where magnetic fields tend to remove all the angular momentum unless they are some how partially dissipated (“magnetic braking catastrophe”), constitute two classical open questions which challenge the ideal MHD description, usually expected to be a good approximation in these environments. Recently, we have investigated an alternative diffusive mechanism based on fast magnetic reconnection induced by turbulence, termed turbulent reconnection diffusion (TRD).
About the second topic, the amplification and maintenance of magnetic fields in the ICM are usually attributed to the turbulent dynamo action. This is generally derived in the collisional MHD framework. However, the ICM requires a collisionless MHD description. Unlike collisional-MHD simulations, our study uses an anisotropic plasma pressure, which brings the plasma within a parameter space where collisionless instabilities should take place. During this talk, I will also discuss the recent results of our 3D numerical simulations of forced turbulence, motivated by modeling the turbulent ICM and understanding the evolution of magnetic fields there.