1. Is solar activity a surface phenomenon?
Axel Brandenburg (Nordita/Stockholm)
Stockholm, 10 April2013
Käpylä+12
Kemel+12
Ilonidis+11
Warnecke+11
Brandenburg+11

2. How deep are sunspots rooted?
Hindman et al. (2009, ApJ)
may not be so deeply rooted
dynamo may be distributed
near-surface shear important
Kosovichev et al. (2000)

3. Sunspot proper motion: rooted at r/R=0.95?
Benevolenskaya, Hoeksema,
Kosovichev, Scherrer (1999)
Pulkkinen & Tuominen (1998)

4. The 4 solar dynamo scenarios
Distributed dynamo (Roberts & Stix 1972)
Positive alpha, negative shear
Overshoot dynamo (e.g. DeLuca & Gilman 1986)
Negative alpha, positive shear
Interface dynamo (Parker 1993)
Negative alpha in CZ, positive radial shear beneath
Low magnetic diffusivity beneath CZ
Flux transport dynamo (Dikpati & Charbonneau 1999)
Positive alpha, positive shear
Migration from meridional circulation

5. Steps toward the overshoot dynamo scenario
Since 1980: dynamo at bottom of CZ
Flux tubes buoyancy neutralized
Slow motions, long time scales
Since 1984: diff rot spoke-like
dW/dr strongest at bottom of CZ
Since 1991: field must be 100 kG
To get the tilt angle right
Spiegel & Weiss (1980)
Golub, Rosner, Vaiana,
& Weiss (1981)

6. Is magnetic buoyancy a problem?
Stratified dynamo simulation in 1990
Expected strong buoyancy losses,
but no: downward pumping
Tobias et al. (2001)

7. Arguments against and in favor?
Tachocline dynamos
Distributed/near-surface dynamo
in favor
Flux storage
Distortions weak
Problems solved with meridional circulation
Size of active regions
Neg surface shear: equatorward migr.
Max radial shear in low latitudes
Youngest sunspots: 473 nHz
Correct phase relation
Strong pumping (Thomas et al.)
against
100 kG hard to explain
Tube integrity
Single circulation cell
Turbulent Prandtl number
Max shear at poles*
Phase relation*
1.3 yr instead of 11 yr at bot
Rapid buoyant loss*
Strong distortions* (Hale’s polarity)
Long term stability of active regions*
No anisotropy of supergranulation
Brandenburg (2005, ApJ 625, 539)

8. 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

9. Ghizaru, Charbonneau, Racine, …
Cycle now common!
Activity from bottom of CZ
but at high latitudes

10. Brun, Brown, Browning, Miesch, Toomre

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

12. Alternative sunspot origins
Kitchatinov & Mazur (2000)
Rogachevskii & Kleeorin (2007)
Brandenburg, Kleeorin , & Rogachevskii (2010)
Stein & Nordlund (2012)

13. Negative effective magnetic pressure instability
Gas+turb. press equil.
B increases
turb. press. decreases
net effect?

14. How can pressure be negative??
Just virtual?
Magnetic buoyancy?
Kemel et al. (2012)
Brandenburg et al. (2011)

15. Predictive power of mean-field approach
DNS
Mean-field simulation (MFS)

16. True instability: exponential growth
Several thousand turnover times
Or ½ a turbulent diffusive time
Exponential growth  linear instability of an already turbulent state

17. NEMPI coupled to dynamo
Explains disappearence
Other problems
Sensitivity to rotation
Nonaxisymmetry?
MFS
Jabbari et al. (2013)
Losada et al. (2013)

18. Broader mean-field concept
a effect, turbulent diffusivity, Yoshizawa effect, etc
Turbulent viscosity and other

19. Conclusions Interest in predicting solar activity
Cyclonic convection ( helicity)
Near surface shear  migratory dynamo
Bi-helical fields, inverse cascade
Solar wind also bi-helical field, but reversed
Formation of active regions and sunspots by negative effective magnetic pressure inst.