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Axel Brandenburg & Jörn Warnecke Nordita Stockholm

Surface appearance of dynamo-generated fields. W loop emergence Buoyant rise Many scale heights Twist needed Dynamo bi-helical field Emergence Twisted arcades. Axel Brandenburg & Jörn Warnecke Nordita Stockholm. Flux emergence: current paradigm.

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Axel Brandenburg & Jörn Warnecke Nordita Stockholm

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  1. Surface appearance of dynamo-generated fields • W loop emergence • Buoyant rise • Many scale heights • Twist needed • Dynamo • bi-helical field • Emergence • Twisted arcades Axel Brandenburg & Jörn Warnecke Nordita Stockholm

  2. Flux emergence: current paradigm • Wrong W gradient in bulk of convection zone • Poleward migration by meridional circulation • Magnetic field in overshoot layer (100 kG) • Flux emergence through Parker’s W loops

  3. Simulations Release of initially untwisted tube of high entropy:  Cusp formation  Flux emergence Formation of coronal loops

  4. Twist to reduce expansion • Expansion is reduced • Strong pitch required • Still no full emergence

  5. Global simulations Y. Fan (Living Reviews 2008)

  6. Coronal mass ejections from helical structures This is how it looks like… Gibson et al. (2002)

  7. How deep are sunspots rooted? • Solar activity may not be so deeply rooted • The dynamo may be a distributed one • Near-surface shear important Hindman et al. (2009, ApJ)

  8. Sunspots shallow phenomena? Theories for shallow spots: (i) Collapse by suppression of turbulent heat flux (ii) Negative pressure effects from <bibj>-<uiuj> vs BiBj

  9. Near-surface shear layer Benevolenskaya, Hoeksema, Kosovichev, Scherrer (1999)

  10. Flux emergence:observations & simulations Brandenburg (2005, ApJ) ~300 gauss mean fields Hindman et al. (2009, ApJ)

  11. Magnetic buoyancy not a problem! Stratified dynamo simulation in 1990 Expected strong buoyancy losses, but no: downward pumping Tobias et al. (2001)

  12. Negative mean-field buoyancy Recent work with Kleeorin & Rogachevskii (arXiv:0910.1835 Astron. Nach. 2010)

  13. Flux emergence: simulations and models • Active regions from an instability • Suppression of turbulent motions • Cooling, contraction, field amplification in preparation with Kleeorin & Rogachevskii

  14. Solar dynamo scenario New loop Differential rotation (surface layers: faster inside) Cyclonic convection; Buoyant flux tubes Equatorward migration  a-effect

  15. How do magnetic helicity losses look like? N-shaped (north) S-shaped (south) (the whole loop corresponds to CME)

  16. Rising tube is bi-helicalsignature of a effect

  17. Hybrid model:dynamo + force-free exterior • Turbulence helically forced, no shear • Large-scale field, bi-helical • Arcade structure with current sheet above neutral line Warnecke & Brandenburg (A&A, arXiv:1002.36201)

  18. Recurrent emergence events •  Exterior nearly force-free • significant J.B in exterior • Recurrent events: Alfven speed • Interval: ~500 Alfven times

  19. Recurrent reconnections

  20. Averaged field: plasmoid ejections Mean field B(y,z,t) Average over x <Bx> color coded <Ax> field lines

  21. Force-free: poloidal ~ toroidal field

  22. Next steps • Study Rm dependence • More realistic solar dynamo • Global model (shell sector) • Transition to solar wind

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