Is solar activity a surface phenomenon?

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

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

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.) Arguments against and in favor? Tachocline dynamos Distributed/near-surface dynamo in favor 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 Losada et al. (2013) Jabbari 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.

Is solar activity a surface phenomenon?