Abstract Details
| Name: Pallavi Bhat Affiliation: PhD student Conference ID: ASI2015_658 Title : Fluctuation dynamos: theory, simulations and observational consequences. Authors and Co-Authors : Prof. Kandaswamy Subramanian, IUCAA Abstract Type : Poster Abstract Category : Extragalactic astronomy Abstract : Turbulence is ubiquitous in many astrophysical systems like galaxies, galaxy clusters and possibly even the filaments in the intergalactic medium. Such turbulent systems host fluctuation dynamos which operate on minimal requirements of the underlying flow, and lead to the exponential growth of magnetic fields on the short eddy turn over time-scales. A random flow with modest magnetic Reynolds number $\Rm\sim 100$ is sufficient to activate the fluctuation dynamo. The only analytical model of the fluctuation dynamo is the Kazantsev model which assumes a velocity field that is delta-correlated in time. We generalize the analytic model of fluctuation dynamo to include the effects of a finite correlation time, $\tau$, using renewing flows. We show, an intriguing result, that to the leading order in $\tau$, the magnetic power spectrum, preserves the Kazantsev form, $M(k) \propto k^{3/2}$, in the large $k$ limit, independent of $\tau$. Such a spectrum peaks at large $k$, thus concentrating most of the magnetic energy at resistive scales. But as the fluctuation dynamo saturates, the Lorentz force tends to shift the energy to larger scales. In order to study this nonlinear evolution, we have studied periodic box simulations of the fluctuation dynamo of high resolution up to $512^3$ and measure the resulting random Faraday rotation measure (RM). We show that when the dynamo saturates, the rms value of RM is of the order of 40–50 per cent of the value expected in a model where fields of strength Brms uniformly fill cells of the largest turbulent eddy but are randomly oriented from one cell to another. We also show that the magnetic integral scale, Lint, which is directly related to the RM dispersion, increases as the dynamo saturates. It appears that due to the ordering effect of the Lorentz forces, Lint of the saturated field tends to a modest fraction, 1/3–1/4 of the integral scale of the velocity field, for all our runs with varying $\Rm$. These results are then applied to discuss the Faraday rotation signatures of fluctuation dynamo generated fields in young galaxies, galaxy clusters and intergalactic filaments. |