Nordita Astrophysics Seminars

Generation of chiral asymmetry by a helical magnetic field [Note: unusual time & day]

by Tomohiro Fujita




It is well known that helical magnetic fields undergo the so-called inverse cascade process in which their correlation length grows due to the magnetic helicity conservation in classical ideal MHD. At high energies above 10MeV, however, classical MHD is necessarily extended to chiral MHD and then the conserved quantity is the magnetic helicity plus chiral chemical potential of charged fermions. We study the evolution of the chiral MHD system with the initial condition of nonzero magnetic helicity and vanishing chiral potential. We derive analytic expressions for their evolution that are compared to a series of laminar and turbulent three-dimensional direct numerical simulations. We find that the late-time evolution of the magnetic helicity depends on the magnetic Reynolds number Rm. For a high Rm where turbulence occurs, the magnetic helicity eventually evolves in the same way as the classical ideal MHD. For a low Rm where the velocity field is negligible, however, the scaling is changed. After being quickly generated, the chiral potential always decays together with the peak wavenumber of the magnetic fields.