带剪切的湍流磁动力学中的 helicity 约束

我们使用强施加剪切的强迫旋流湍流模拟来研究磁场的演化。在一些初始的指数增长之后，磁场发展出大尺度传播波模式。 resulting field structure possesses magnetic helicity, which is conserved in a periodic box by the ideal MHD equations and can hence only change on a resistive time scale. This constrains strongly the growth time of the large scale magnetic field, but less strongly the length of the cycle period. Comparing with the case without shear, the time scale for large scale field amplification is shortened by a factor Q, which depends on the relative importance of shear and helical turbulence, and which controls also the ratio of toroidal to poloidal field. The results of the simulations can be reproduced qualitatively and quantitatively with a mean-field alpha-Omega dynamo model with alpha-effect and the turbulent magnetic diffusivity coefficients that are less strongly quenched than in the corresponding alpha^2-dynamo.