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Is solar activity a surface phenomenon?. Axel Brandenburg (Nordita/Stockholm). Kemel+12. K äpylä +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).

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is solar activity a surface phenomenon
Is solar activity a surface phenomenon?

Axel Brandenburg (Nordita/Stockholm)






how deep are sunspots rooted
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
Sunspot proper motion: rooted at r/R=0.95?

Benevolenskaya, Hoeksema,

Kosovichev, Scherrer (1999)

Pulkkinen & Tuominen (1998)

the 4 solar dynamo scenarios
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
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
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
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


  • 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
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
Ghizaru, Charbonneau, Racine, …
  • Cycle now common!
  • Activity from bottom of CZ
  • but at high latitudes
dynamo wave from simulations
Dynamo wave from simulations

Kapyla et al (2012)

a lternative sunspot origins
Alternative sunspot origins

Kitchatinov & Mazur (2000)

Rogachevskii & Kleeorin (2007)

Brandenburg, Kleeorin , & Rogachevskii (2010)

Stein & Nordlund (2012)

negative effective magnetic pressure instability
Negative effective magnetic pressure instability
  • Gas+turb. press equil.
  • B increases
  • turb. press. decreases
  • net effect?
how can pressure be negative
How can pressure be negative??
  • Just virtual?
  • Magnetic buoyancy?

Kemel et al. (2012)

Brandenburg et al. (2011)

predictive power of mean field approach
Predictive power of mean-field approach


Mean-field simulation (MFS)

true instability exponential growth
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
NEMPI coupled to dynamo
  • Explains disappearence
  • Other problems
    • Sensitivity to rotation
    • Nonaxisymmetry?


Losada et al. (2013)

Jabbari et al. (2013)

broader mean field concept
Broader mean-field concept

a effect, turbulent diffusivity, Yoshizawa effect, etc

Turbulent viscosity and other

  • 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.