Solar activity as a surface Solar activity as a surface phenomenonphenomenon
Axel Brandenburg (Nordita/Stockholm)
Kemel+12Kemel+12 Ilonidis+11Ilonidis+11 Brandenburg+11Brandenburg+11Warnecke+11Warnecke+11KKäpylääpylä+12+12
The thin flux tube paradigm
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Caligari et al. (1995) Charbonneau & Dikpati (1999)
Spruit paper
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Standard dynamo waveStandard dynamo wave
Differential rotation(faster inside) Cyclonic convection;
Buoyant flux tubesEquatorward
migration
New loop
-effect
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Simulations of the solar dynamo?Simulations of the solar dynamo?
• Tremendous stratification– Not only density, also scale height change
• Near-surface shear layer (NSSL) not resolved• Contours of 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
Brun, Brown, Browning, Miesch, ToomreBrun, Brown, Browning, Miesch, Toomre
6Brown et al. (2011)ASH code: anelastic
spherical harmonics
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• Cycle now common!
• Activity from bottom of CZ
• but at high latitudes
Ghizaru, Ghizaru, Charbonneau, Charbonneau,
Racine, …Racine, …
Racine et al. (2011)
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Dynamo wave from simulations Kap
yla et al (2012)
Type of Type of dynamo?dynamo?
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• Use phase relation
• Closer to 2 dynamo
• Wrong for dyn.
Mitra et al. (2010)
Oscillatory 2 dynamo
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Turbulent sunspot origins?Turbulent sunspot origins?
Theories for shallow spots:Theories for shallow spots:
(i) Collapse by suppression(i) Collapse by suppressionof turbulent heat fluxof turbulent heat flux
(ii) Negative pressure effects(ii) Negative pressure effectsfrom <from <uuiiuujj> vs > vs BBiiBBjj
Kosovichev et al. (2000)
Kosovichev et al. (2000)
Turbulent sunspot origins?Turbulent sunspot origins?
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Negative effective magnetic pressure instability
• Gas+turb. press equil.
• B increases
• Turb. press. Decreases
• Net effect?
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Much stronger with vertical fields
• Gas+turb. press equil.
• B increases
• Turb. press. Decreases
• Net effect?
Self-assembly of a magnetic spot• Minimalistic model• 2 ingredients:
– Stratification & turbulence
• Extensions– Coupled to dynamo– Compete with rotation– Radiation/ionization
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Sunspot decay
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5123 vs 10243 resolution
• Rm/Re dependence?
• Here 40/80 and 95/190
• Originally 18/36.
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Surface-filling magnetic activity
17Guedel (2004)Saturated activity naturally explained
3 times stronger stratification
• Rm/Re dependence?
• Here 40/80 and 95/190
• Originally 18/36.
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Imposed vs. self-assembly• Appearance of sunspot
when coupled to radiation
• Can be result of self-assembly when ~1000 G field below surface
19Stein & Nordlund (2012)
Rempel et al. (2009)
Why so strong?
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Vertical fields survive downward flow
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ConclusionsConclusions• 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.