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

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)

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

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

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)

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

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)

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

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

Brun, Brown, Browning, Miesch, Toomre

Dynamo wave from simulations Kapyla et al (2012)

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

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

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

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

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

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

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

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.