Monday, November 26, 2012

Papers: Magnetic Wreaths and Cycles in Convective Dynamos

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  • Nelson, Nicholas J., Brown, Benjamin P., Brun, Allan Sacha, Miesch, Mark S., & Toomre, Juri. 2013. "Magnetic Wreaths and Cycles in Convective Dynamos", The Astrophysical Journal, 762, 73
This paper discussed a series of 3D dynamo simulations with ASH of sun-like stars. These dynamos achieved magnetic wreaths - bands of primarily longitudinal magnetic field in each hemisphere. Ben Brown's work had previously shown that persistent wreaths could become cyclic with increased rotation rate.  Here Ben and I used a series of simulations to show that cycles can also occur when simulations are made more turbulent at a fixed rotation rate.

Three ASH simulations which were the focus of this paper: D3 (left), D3a (center), and D3b (right). The simulations are identical except in their diffusion. D3a is about twice as turbulent as D3, and D3b is about twice again as turbulent. From top to bottom, the panels show radial velocities near the top of each simulation, longitudinal magnetic fields at mid-convection zone, and 3D volume renderings of magnetic field lines near the equator colored by longitudinal magnetic field.
In addition to the onset of cycles, we also showed that as these simulations become more turbulent a number of fundamental balances change. The transport of angular momentum which supports differential rotation in D3 is a balance between Reynold's stresses and viscous diffusion, but in D3b diffusion has been replaced by magnetic stresses.  The wreaths in D3 are dissipated primarily by resistive diffusion, but in D3b resolved turbulence has assumed the primary role.  Finally, we showed that the cycles themselves are caused by a breakdown in the balance between turbulent correlations and diffusion in maintaining the poloidal magnetic field.
Cartoon diagram of how cycles operate in case D3b. Magnetic wreaths (a) lead to an electromotive force (b) via an alpha-like effect, which generates poloidal magnetic field (c) through induction, which generates wreaths of the opposite polarity (d) through the Omega effect.
Finally, we showed that the so-called "alpha" effect which closes the loop on our cycles has the same timescale as convection.