Magnetic field Amplification in a Rotating Astrophysical Plasma Sphere: $α$ and $β$ Effects

Abstract: We investigated the generation of the $\alpha$ and $\beta$ effects in a rotating spherical plasma system with oppositely polarized kinetic helicity in the northern and southern hemispheres and examined their contributions to the induction of magnetic fields. We found that the $\alpha$ effect is relatively small, and its sign depends on the polarization of kinetic helicity. In contrast, the $\beta$ effect remains negative regardless of the sign of kinetic helicity. Despite its small magnitude, the $\alpha$ effect plays a crucial role in determining the polarity of helical magnetic structures, while a negative $\beta$ indicates energy diffusion from turbulent regions into the large-scale magnetic field. We derived the $\alpha$ and $\beta$ effects with oppositely polarized kinetic helicity using different approaches, incorporating large-scale magnetic data and turbulent kinetic data. These were used to reproduce the large-scale magnetic field and compare it with DNS results. In the kinematic regime, where the magnetic field strength is weak, our results align well; however, in regions with strong nonlinear magnetic effects, the magnetic field reproduced using turbulent kinetic data diverges. This divergence is attributed to insufficient quenching of the $\beta$ effect, suggesting that including the second-moment terms of velocity in the magnetic field effect would improve the accuracy of the $\beta$ coefficient. In this study, we considered the case of a rotating plasma sphere with $Pr_M = 1$ and low Reynolds numbers. However, in reality, Reynolds numbers are much higher, and $Pr_M$ is much less than 1, which necessitates further studies on this topic. We plan to address this in future research.