Publication:

(1) Magnetic Self-organization in Convective Dynamos

Figure 1: Magnetic self-organization and polarity reversals in numerical simulations of solar and stellar convection.The top and bottom row represent solar-type stars rotating at the solar rate (rotation period 28 days) and five times faster (rotation period 5.6 days).(A) Vertical velocity near the top of the convection zone (yellow/orange denotes upflow and blue/black downflow). (B) Angular velocity averaged over longitude and time (pink denotes faster rotation, blue/black slower). (C) Mean (longitudinally-averaged) toroidal magnetic field (red and blue denote eastward and westward flux; peak field strengths range from several kG on the top to about 20–30 kG on the bottom). (D) Time evolution of the mean toroidal field at the base of the convection zone, illustrating cyclic polarity reversals.

The challenge for solar dynamo theory is to determine how ordered patterns such as the 22-year magnetic activity cycle arise from the highly turbulent, chaotic conditions of the solar convection zone. The generation of coherent large-scale magnetic flux from smaller-scale turbulent motions is intimately linked to rotational shear and helicity in flows and magnetic fields. Numerical simulations of global-scale solar convection, presented in Figure 1, are shedding new light on this fundamental issue. As the rotation rate is increased from the solar value to a rate that is five times faster, which is common in younger solar-like stars, the scale of convective patterns decreases. Rotation enhances the agents of magnetic self-organization, helicity and rotational shear, which in turn create prominent toroidal magnetic structures. In solar parameter regimes these are pumped to the bottom of the convection zone but they may subsist in the midst of the convection zone in rapidly rotating younger stars. As time proceeds, the toroidal bands reverse polarity in a quasi-cyclic fashion. The cycle period is generally shorter for rapid rotators but exhibits a complex parameter dependence that researchers are now investigating further. Such simulations demonstrate that magnetic self-organization and cyclic variability are common features in global convective dynamos and have important implications for understanding the dynamic origins of solar and stellar magnetic activity.

Acknowledgements: This work is partially supported by NASA's Heliophysics Supporting Research & Technology grant NNH09AK14I and NASA's Heliophysics Theory Program grant NNX08AI57G.

Team: Mark Miesch (HAO/NCAR), Benjamin Brown (Center for Magnetic Self-Organization, Unversity of Wisconsin) Matthew Browning (Canadian Institute for Theoretical Astrophysics, University of Toronto, Canada) Allan Sacha Brun (Commissariat a l'Energie Atomique, Sacay, France) Juri Toomre, Nicholas Nelson, Nicholas Featherstone, Kyle Augustson (JILA and Department of Astrophysical and Planetary Sciences, University of Colorado, Boulder, CO)