Benjamin Brown

 
 


Dynamical Stellar Interiors

I study dynamical processes occurring in stellar interiors.  In particular, I’m interested in convection and dynamo action in stars like our sun. I explore the processes behind solar and stellar dynamo action with three-dimensional (3-D) magnetohydrodynamic (MHD) simulations of stellar convection, generally using the anelastic spherical harmonic (ASH) code. I’m very interested in how rotation affects the convection and dynamo action realized in stars like our Sun. Through these simulations, we have found that highly-organized wreath-like magnetic structures can arise in the midst of the turbulent convection zone for stars that rotate more rapidly than the Sun. The persistence of these magnetic wreaths has been a great surprise. Wreath-building dynamos can cyclically reverse their global-scale magnetic fields, and this is causing us to rethink aspects of dynamo theory.


I am an NSF Astronomy and Astrophysics Postdoctoral Fellow (AAPF) at the University of Wisconsin, Madison.

 

Magnetic wreaths in the convection zone of a solar-type star (case D5).  Movies here.

Select Publications

(Full list [19 articles])


  1. 1.Energy Conservation and Gravity Waves in Sound-Proof Treatments of Stellar Interiors: Part I Anelastic Approximations, Brown, B P, Vasil G M, Zweibel E G, 2012, ApJ, 756, 109:1-20

  2. 2.Magnetic Cycles in a Convective Dynamo Simulation of a Young Solar-type Star, Brown, B P, Miesch, M S, Browning, M K, Brun, A S, & Toomre, J, 2011, ApJ, 731, 69:1–19

  3. 3.Persistent magnetic wreaths in a rapidly rotating sun, Brown, B P, Browning, M K, Brun, A S, Miesch, M S, & Toomre, J, 2010, ApJ, 711, 424–438

  4. 4.Exploring the Pcyc vs. Prot relation with flux transport dynamo models of solar-like stars, Jouve, L, Brown, B P, & Brun, A S, 2010, A&A, 509, A32:1–11

  5. 5.Rapidly rotating suns and active nests of convection, Brown, B P, Browning, M K, Brun, A S, Miesch, M S, & Toomre, J, 2008, ApJ, 689, 1354–1372

Solar and Stellar Dynamos


In stars like our Sun, magnetic fields seen at the surface are generated by dynamo action in the turbulent sub-photospheric stellar convection zone. There, turbulent plasma motions driven by convection couple with rotation and magnetic fields to build and rebuild the magnetic fields, sustaining them against ohmic decay. This process, the conversion of kinetic energy to magnetic energy, is the fundamental dynamo process.


Despite the Sun being the most turbulent nearby object, solar magnetism is remarkably organized. At the surface we see coherent magnetic structures called sunspots. We are unsure of their origin, but it is likely that these Earth-sized magnetic tubes are tied to magnetic fields which thread deep into the convection zone. They may connect all the way to the very bottom of the convection zone to the tachocline, a special interface layer between the convection zone and the stable radiative interior.


Sunspots are not static; rather they are ever changing. An individual sunspot lasts for about one month before decaying away, spread by the turbulent mixing of small-scale surface convection in the solar photosphere (granulation and supergranulation). This is very similar to the rotation period of the Sun, so generally we only see a sunspot once. Sunspots emerge continually at the surface, waxing and waning in numbers during the eleven-year solar cycle. Sunspots and the solar cycle must have a common origin in the solar convection zone, but the solar dynamo remains a mystery despite more than a century of astronomical study.


I study solar and stellar dynamo action with three-dimensional   (3-D) magnetohydrodynamic (MHD) simulations of stellar convection. Generally, these simulations are conducted with the anelastic spherical harmonic (ASH) code.  These stellar convection simulations are global in scale, capturing a spherical shell of rotating and stratified plasma with properties similar to the solar convection zone.  For the Sun, helioseismically tested 1-D stellar structure models give us a good starting place. For other stars we can build similar (but as yet less-tested) models using stellar structure codes like the MESA code.  Simulations of stellar convection require substantial computational resources and typically run on massively-parallel supercomputers with NSF and NASA support. Currently most of our computing is conducted on Ranger at TACC, Kraken at NICS and Pleiades at NASA.  With resources such as these, we are now at a point where reasonably turbulent convection can be achieved in a variety of stars.

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ArXiV author page

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