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differential rotation in D3 is a balance between Reynold's stresses and viscous diffusion, but in D3b diffusion has been replaced by magnetic stresses.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.

Papers: Buoyant Magnetic Loops in a Global Dynamo Simulation of a Young Sun

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• Nelson, Nicholas J., Brown, Benjamin P., Brun, Allan Sacha, Miesch, Mark S., & Toomre, Juri. 2011. "Buoyant Magnetic Loops in a Global Dynamo Simulation of a Young Sun", The Astrophysical Journal Letters, Volume 739, Issue 2, L38

Longitudinal magnetic field as a function of radius and latitude at successive times. Two buoyant magnetic structures are captured here,

 This letter introduced a dynamo simulation of a sun-like star which produced buoyant magnetic loops.  The simulation, which we call case S3, uses an improved treatment of diffusion which allowing the simulation to be much more turbulent than was previously possible.

Papers: Global magnetic cycles in rapidly rotating younger suns

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• Nelson, Nicholas J., Brown, Benjamin P., Browning, Matthew K., Brun, Allan Sacha, Miesch, Mark S., & Toomre, Juri. 2011, "Global magnetic cycles in rapidly rotating younger suns", The Physics of Sun and Star Spots, Proceedings of the International Astronomical Union, IAU Symposium, Volume 273, p. 272-275

This paper was based on a conference talk given at the IAU Symposium #273: The Physics of Sun and Star Spots in Ventura, California in August 2010. I ran and analysed all but one of the simulations here.  Ben Brown was instrumental in developing many of the analysis techniques used. Mark, Sacha, and Juri played key roles in the formulation and analysis of the simulations. This was the first presentation of buoyant magnetic loops in ASH simulations.

Papers: Strong Dynamo Action in Rapidly Rotating Suns

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• Brown, Benjamin P., Browning, Matthew K., Brun, Allan Sacha, Miesch, Mark S., Nelson, Nicholas J., & Toomre, Juri, 2007, "Strong Dynamo Action in Rapidly Rotating Suns", in UNSOLVED PROBLEMS IN STELLAR PHYSICS: A Conference in Honor of Douglas Gough. AIP Conference Proceedings, Volume 948, pp. 271-27

My contribution was in running and analysing one of the three simulations presented here.