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Rethinking

Rethinking basic concepts of solar convection and sunspot formation. Rethinking. Axel Brandenburg (Nordita, Stockholm). Spaceweather.com. SpaceWeather.com. Movie of the Sun. X-ray corona. X-ray corona. Triggers geomagnetic storms Aviation: affects communication & GPS

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Rethinking

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  1. Rethinking basic concepts of solar convection and sunspot formation Rethinking Axel Brandenburg (Nordita, Stockholm)

  2. Spaceweather.com SpaceWeather.com

  3. Movie of the Sun

  4. X-ray corona X-ray corona Triggers geomagnetic storms Aviation: affects communication & GPS Harmful proton radiation (~mSv)

  5. Structure of the Sun Surface: granulation (~1Mm) Radius of the Sun: 700 Mm Convection zone: 200 Mm

  6. Agreement: simulations & observations Simulation: Stein & Nordlund, observation Swedish Solar Telescope What about deeper down?

  7. Structure of convection zonemixing length theory vs simulations

  8. Hanasoge

  9. Results challenged • Ring diagram analysis by Greer et al. (2015) • One difference: no “noise” removed • Kernels 

  10. Basic concept ofhelioseismology Top: reflection when wavenlength ~ density scale height Deeper down: Sound speed large

  11. Travel time differences • Contrib. from whole path • Esp. top layers (cs small) •  averaging over rays through same point

  12. Deep-focusing geometry • Removes strong contributions from top layers • Could they be right?

  13. Other reasons for concern • Simulations predict giant cells • But are not observed

  14. Do we need to rethink? • In mixing length theory: l=Hp only hypothesis • Simulations: subgrid scale diffusion, viscosity • Envisage reasons for (i) smaller scale flows and/or (ii) deeper parts subadiabatic? • Convection zone still 200 Mm

  15. Helioseismology: change at 0.7R

  16. Spruit97 A changing paradigm

  17. Entropy rain

  18. Stein & Nordlund (1998) simulations Filamentary, nonlocal shown: entropy fluctuationsposneg

  19. Entropy & convection Adiabatic changes: S=const P equilibrium: S+  buoyant S pert overshoot z pert unstable S z

  20. Tau approximation Closure hypothesis

  21. Deardorff1

  22. Deardorff2

  23. Physical meaning? pert coasting… S z

  24. Why should only the top be unstable e.g. if Power law Polytropic index n

  25. Deeper parts intrinsically stable n=3.25 Kramers opacity (interior): a=1, b=-7/2 Polytropic index n Entropy gradient positive (stable) for n > 3/2

  26. Early work in the 1930s

  27. Why should only the top be unstable

  28. Hydrostaticreferencesolutions Thickness only ~1Mm

  29. Revised mixing length theory Entropy gradient old  new

  30. New solutionswith Deardorffflux

  31. Consequences of small scales • Larger kf  less turb. Diffusion: ht=urms/3kf • Applications to dynamos: stronger, less turb diffusive • Two other important effect: • Lambda effect  differential rotation • Negative effective magnetic pressure  spots

  32. Negative effective magnetic pressure instability Kleeorin, Rogachevskii, Ruzmaikin (1989, 1990) • Gas+turbulent+magnetic pressure; in pressure equil. • B increases  turbulence is suppressed •  turbulent pressure decreases • Net effect?

  33. Setup • 3-D box, size (2p)3, isothermal MHD • Random, nonhelical forcing at kf/k1=5, 15 or 30 • Stratified in z, r~exp(-z/H), H=1, Dr=535 • Periodic in x and y • stress-free, perfect conductor in z • Weak imposed field B0 in y • Run for long times: what happens? • Turnover time tto=(urmskf)-1, turb diff ttd=(htk12)-1 • Is longer by factor 3(kf/k1)2 = 3 152 = 675 • Average By over y and Dt=80tto

  34. Basic mechanism Anelastic: descending structure  compression B amplifies B amplifies Growth rate

  35. Self-assembly of a magnetic spot • Minimalistic model • 2 ingredients: • Stratification & turbulence • Extensions • Coupled to dynamo • Compete with rotation • Radiation/ionization

  36. Sunspot formation that sucks Mean-field simulation: Neg pressure parameterized Typical downflow speeds Ma=0.2…0.3 Brandenbur et al (2014)

  37. Flux tubes in global simulations Nelson, Brown, Brun, Miesch, Toomre (2014)

  38. Other proposals • Rising flux tubes? • Hierachical convection? • Self-organization as part of the dynamo g.B  u.B g.W u.w  A.B

  39. Bi-polar regions in simulations with corona Warnecke et al. (2013, ApJL 777, L37)

  40. First dynamo-generated bi-polar regions Mitra et al. (2014, arXiv)

  41. Global models Jabbari et al. (2015, arXiv)

  42. Conclusions • Sun: active & exciting • Some basic questions worth rethinking • Possibly Deardorff flux (Entropy rain) •  slightly subadiabatic: no giant cells • Other interesting possibilities: dynamos, differential rotation, spotformation, …

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