Identification of vortexes obstructing the dynamo mechanism in laboratory experiments

The magnetohydrodynamic dynamo effect explains the generation of self-sustained magnetic fields in electrically conducting flows, especially in geo- and astrophysical environments. Yet the details of this mechanism are still unknown, e.g., how and to which extent the geometry, the fluid topology, the forcing mechanism and the turbulence can have a negative effect on this process. We report on numerical simulations carried out in spherical geometry, analyzing the predicted velocity flow with the so-called Singular Value Decomposition, a powerful technique that allows us to precisely identify vortexes in the flow which would be difficult to characterize with conventional spectral methods. We then quantify the contribution of these vortexes to the growth rate of the magnetic energy in the system. We identify an axisymmetric vortex, whose rotational direction changes periodically in time, and whose dynamics are decoupled from those of the large scale background flow, is detrimental for the dynamo effect. A comparison with experiments is carried out, showing that similar dynamics were observed in cylindrical geometry. These previously unexpected eddies, which impede the dynamo effect, offer an explanation for the experimental difficulties in attaining a dynamo in spherical geometry.Comment: 25 pages, 12 figures, submitted to Physics of Fluid

Radio emission from dark matter annihilation in the Large Magellanic Cloud

The Large Magellanic Cloud, at only 50 kpc away from us and known to be dark matter dominated, is clearly an interesting place where to search for dark matter annihilation signals. In this paper, we estimate the synchrotron emission due to WIMP annihilation in the halo of the LMC at two radio frequencies, 1.4 and 4.8 GHz, and compare it to the observed emission, in order to impose constraints in the WIMP mass vs. annihilation cross section plane. We use available Faraday rotation data from background sources to estimate the magnitude of the magnetic field in different regions of the LMC's disc, where we calculate the radio signal due to dark matter annihilation. We account for the e+ e- energy losses due to synchrotron, Inverse Compton Scattering and bremsstrahlung, using the observed hydrogen and dust temperature distribution on the LMC to estimate their efficiency. The extensive use of observations, allied with conservative choices adopted in all the steps of the calculation, allow us to obtain very realistic constraints.Comment: 9 pages, 7 figure

Numerical Simulation of Helium Arc at High Pressure and Low Current

International audienc