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Rethinking Basic Concepts of Solar Convection and Sunspot Formation by Axel Brandenburg

Explore the latest insights into solar convection and sunspot formation by renowned researcher Axel Brandenburg. Delve into the structure of the Sun, convection zones, and the impacts on space weather and aviation. Challenge traditional theories and discover new perspectives through simulations and observations.

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Rethinking Basic Concepts of Solar Convection and Sunspot Formation by Axel Brandenburg

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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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