AR_final file_2018-19

with large-scale dynamos. However, the possibil- ity of small-scale dynamos being excited at small and intermediate ratios of viscosity to magnetic dif- fusivity (the magnetic Prandtl number) has been debated, and the possibility of them depending on the large-scale forcing wavenumber has been raised. Axel Brandenburg, N. E. L. Haugen, Xiang-Yu Li, and Kandaswamy Subramanian showed, using four values of the forcing wavenumber, that the small-scale dynamo does not depend on the scale- separation between the size of the simulation do- main and the integral scale of the turbulence, i.e., the forcing scale. Moreover, the spectral bottleneck in turbulence, which has been implied as being re- sponsible for raising the excitation conditions of small-scale dynamos, is found to be invariant un- der changing the forcing wavenumber. However, when forcing at the lowest few wavenumbers, the effective forcing wavenumber that enters in the def- inition of the magnetic Reynolds number is found to be about twice the minimum wavenumber of the domain. This work is relevant to future studies of small-scale dynamos, of which several applications are being discussed. Turbulent transport coefficients in galactic dynamo simulations using singular value decomposition Coherent magnetic fields in disc galaxies are thought to be generated by a large-scale (or mean-field) dynamo operating in their interstellar medium (ISM). A key driver of mean-field growth is the turbulent electromotive force (EMF), which represents the influence of small-scale velocity and magnetic fields on the mean-field. This is usually expressed as a linear expansion in the mean-field and its derivatives, with dynamo coefficients as expansion coefficients. Abhijit B. Bendre , Kandaswamy Subramanian , Detlef Elstner, and Oliver Gressel have adopted the singular value decomposition (SVD) method to directly measure these dynamo or turbulent transport coefficients in a simulation of the turbulent ISM that realizes a large-scale dynamo. Specifically, the SVD is used to least square fit the time series data of EMF with that of mean-field and its derivatives, to determine these coefficients. They demonstrate that the profiles of reconstructed EMF with SVD match well with that obtained directly from the simulation. Also as a direct test, they use these coefficients to simulate a 1-D dynamo model and find an overall similarity in the evolution of the mean-field between the dynamo model and the direct simulation. They also compare the results with that obtained previously using the test-field method and find reasonable agreement. Over- all, the SVD method provides an effective post processing tool to determine turbulent transport coefficients from simulations. (see Figures 9 and 10). Efficient quasi-kinematic large-scale dynamo as the small-scale dynamo saturates Large-scale magnetic fields in stars and galaxies are thought to arise by mean-field dynamo action due to the combined influence of both helical turbu- lence and shear. Those systems are also highly conducting, where turbulence leads to a fluctua- tion (or small-scale) dynamo, which more rapidly amplifies magnetic field fluctuations on the eddy scales and smaller. Will this then interfere with and suppress the mean (or large-scale) field growth? Using direct numerical simulations of helical tur- bulence (with and without shear), Pallavi Bhat, Kandaswamy Subramanian, and Axel Branden- burg identify a novel quasi-kinematic large-scale dy- namo, which operates as the small-scale dynamo saturates. Thus, both dynamos operate efficiently, one after the other, and lead to the generation of significant large-scale fields. The origin of large-scale magnetic fields in low-mass galaxies The origin of large-scale magnetic fields, detected in some low-mass (dwarf and irregular) galaxies via polarised synchrotron emission and Faraday rota- tion, remained unexplained for a long time. Pras- anta Bera, Anvar Shukurov, and Kandaswamy Subramanian suggest that mean-field dynamo ( 71 )