Dynamo Action in Rotating and Shearing Flows

Abstract

Magnetic fields are omnipresent in universe. They play important role in the formation of stars, galaxies and the accretion onto compact objects and the collimation of the relativistic jets of radio galaxies. It is intriguing to understand the origin of magnetic fields in such objects, which has scales comparable to the size of the system and ranges from micro-Gauss to kilo-Gauss strength. In the canonical dynamo theory, it is been shown that if fluid posses kinetic helicity on the average, it would lead to the generation of magnetic field at larger scales. Although, such a net helicity cannot be expected in all astrophysical settings. In particular, disc galaxies have sub-critical strength of kinetic helicity needed to trigger the instability to generate the magnetic field. Hence, the quest for alternative dynamo mechanism. Therefore we considered zero-mean helicity fluctuations in the background of linear shear flows as a alternate kinematic dynamo mechanism. We consider both spatial and temporal zero-mean helicity fluctuations in the background of shear flows. Pure temporal fluctuations are considered in the renovating flow set-up, where we show the growth of large scale magnetic field without the need of net negative diffusion, as diffusivity is found to be positive in the numerical simulations. Interesting behaviour of growth rates and the cycle period of the dynamo waves at high shear has implication for the magnetic activity cycle observed in the slow and fast rotating stars. Spatial anisotropy of the helicity fluctuations are considered in the mean-field setting, where it is shown that the gradients of the fluctuations enters as a velocity, causing the Doppler-shift of the phase of the dynamo wave in the delta-correlated fluctuations. Whereas, in case of finite time correlated fluctuations, this large-scale gradients in the helicity fluctuations leads to the interesting large scale dynamo action, with growth varying linearly with shear.