Axel Brandenburg & Jörn Warnecke NorditaStockholm  loop emergence –Buoyant rise –Many scale heights –Twist needed Dynamo –bi-helical field Emergence –Twisted arcades Surface appearance of dynamo-generated fields

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

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

8 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)

9 Sunspots shallow phenomena? Theories for shallow spots: (i) Collapse by suppression of turbulent heat flux (ii) Negative pressure effects from - vs B i B j

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

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

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

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

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

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

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

17 Rising tube is bi-helical signature of  effect

18 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: )

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

20 Recurrent reconnections

21 Averaged field: plasmoid ejections Mean field B(y,z,t) Average over x color coded field lines

22 Force-free: poloidal ~ toroidal field

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