Solar activity as a surface phenomenon

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

2

Caligari et al. (1995) Charbonneau & Dikpati (1999)

Spruit paper

3

4

Standard dynamo waveStandard dynamo wave

Differential rotation(faster inside) Cyclonic convection;

Buoyant flux tubesEquatorward

migration

New loop

-effect

5

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

7

• Cycle now common!

• Activity from bottom of CZ

• but at high latitudes

Ghizaru, Ghizaru, Charbonneau, Charbonneau,

Racine, …Racine, …

Racine et al. (2011)

8

Dynamo wave from simulations Kap

yla et al (2012)

Type of Type of dynamo?dynamo?

9

• Use phase relation

• Closer to 2 dynamo

• Wrong for dyn.

Mitra et al. (2010)

Oscillatory 2 dynamo

10

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?

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?

Self-assembly of a magnetic spot• Minimalistic model• 2 ingredients:

– Stratification & turbulence

• Extensions– Coupled to dynamo– Compete with rotation– Radiation/ionization

14

Sunspot decay

15

5123 vs 10243 resolution

• Rm/Re dependence?

• Here 40/80 and 95/190

• Originally 18/36.

16

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.

18

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?

20

Vertical fields survive downward flow

21

22

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.