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Solar activity as a surface phenomenon

Solar activity as a surface phenomenon. Axel Brandenburg (Nordita/Stockholm). Kemel+12. K äpylä +12. Ilonidis+11. Warnecke+11. Brandenburg+11. The thin flux tube paradigm. Caligari et al. (1995). Charbonneau & Dikpati (1999). Spruit paper. Standard dynamo wave. New loop.

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Solar activity as a surface phenomenon

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  1. Solar activity as a surface phenomenon Axel Brandenburg (Nordita/Stockholm) Kemel+12 Käpylä+12 Ilonidis+11 Warnecke+11 Brandenburg+11

  2. The thin flux tube paradigm Caligari et al. (1995) Charbonneau & Dikpati (1999)

  3. Spruit paper

  4. Standard dynamo wave New loop Differential rotation (faster inside) Cyclonic convection; Buoyant flux tubes Equatorward migration  a-effect

  5. Simulations of the solar dynamo? • Tremendous stratification • Not only density, also scale height change • Near-surface shear layer (NSSL) not resolved • Contours of W cylindrical, not spoke-like • (i) Rm dependence (catastrophic quenching) • Field is bi-helical: to confirm for solar wind • (ii) Location: bottom of CZ or distributed • Shaped by NSSL (Brandenburg 2005, ApJ 625, 539) • Formation of active regions near surface

  6. Brun, Brown, Browning, Miesch, Toomre ASH code: anelastic spherical harmonics Brown et al. (2011)

  7. Ghizaru, Charbonneau, Racine, … • Cycle now common! • Activity from bottom of CZ • but at high latitudes Racine et al. (2011)

  8. Dynamo wave from simulations Kapyla et al (2012)

  9. Type of dynamo? • Use phase relation • Closer to a2 dynamo • Wrong for aW dyn. Oscillatory a2 dynamo Mitra et al. (2010)

  10. Turbulent sunspot origins? Kosovichev et al. (2000) Theories for shallow spots: (i) Collapse by suppression of turbulent heat flux (ii) Negative pressure effects from <uiuj> vs BiBj

  11. Turbulent sunspot origins?

  12. Negative effective magnetic pressure instability • Gas+turb. press equil. • B increases • Turb. press. Decreases • Net effect?

  13. Much stronger with vertical fields • Gas+turb. press equil. • B increases • Turb. press. Decreases • Net effect?

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

  15. Sunspot decay

  16. 5123 vs 10243 resolution • Rm/Re dependence? • Here 40/80 and 95/190 • Originally 18/36.

  17. Surface-filling magnetic activity Guedel (2004) Saturated activity naturally explained

  18. 3 times stronger stratification • Rm/Re dependence? • Here 40/80 and 95/190 • Originally 18/36.

  19. Imposed vs. self-assembly • Appearance of sunspot when coupled to radiation • Can be result of self-assembly when ~1000 G field below surface Rempel et al. (2009) Stein & Nordlund (2012)

  20. Why so strong?

  21. Vertical fields survive downward flow

  22. Conclusions • Interest in predicting solar activity • Cyclonic convection ( helicity) • Near surface shear  migratory dynamo? • Formation of active regions and sunspots by negative effective magnetic pressure inst.

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