Solar activity as a surface phenomenon
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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
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The thin flux tube paradigm
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Caligari et al. (1995) Charbonneau & Dikpati (1999)
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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
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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)
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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)
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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?
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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
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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)
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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.