Solar Surface Magneto-Convection and Dynamo Action

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Presentation transcript:

Solar Surface Magneto-Convection and Dynamo Action Bob Stein Michigan State University East Lansing, MI, USA Åke Nordlund Niels Bohr Institute for Astronomy, Physics, and Geophysics University of Copenhagen, DK

Weak Field Simulations Initial conditions Snapshot of granular convection (6x6x3 Mm) Resolution: 25 km horizontally, 15-35 km vertically 1G or 30G horizontal seed field imposed Boundary conditions Top boundary: B  potential field Bottom boundary: inflows advect 1G or 30G horizontal field Results Flux emergence, convergence, fragmentation, cancellation, submergence

3D Effects Inhomogeneous T (see only cool gas), Pturb Raises atmosphere 1 scale height

Magneto-Convection

Simulation Results: B Field lines

Flux Emergence & Disappearance

Field Distribution

Exponential Distribution

Exponential Distribution

Exponential Distribution

Exponential Distribution

Exponential Distribution

Strong “Flux Tubes” are dark, depressed & suppress flow

Stokes profiles new SVST, perfect seeing

Stokes Image - Quiet Sun Synthetic Observation - La Palma Telescope MTF + Excellent Seeing Stokes V Surface Intensity 6 Mm 6 Mm

Stokes Image - Quiet Sun Synthetic Observation - Perfect Telescope & Seeing Stokes V Surface Intensity 6 Mm 6 Mm

Magnetic “Flux Tubes” (top view)

Magnetic “Flux Tubes” (side view)

Magnetic field lines

Dynamo Action

Is there a local surface dynamo here? Flux Emergence Less cycle variation slope = -2 Cycle variation

Dynamo Requirements Magnetic field amplification by stretching and twisting Diffusion to reconnect magnetic field lines and alter magnetic topology Circulation to continue process

Magnetic Field Reorganization

Field Amplification Field is concentrated by diverging upflows Field is stretched and twisted by vortical downflows

Vorticity

Work & Dissipation

Stratified convective flows have little re-circulation

Stratification Pumping Dorch & Nordlund (2000) Initial field Most of the flux is INSIDE the convection zone Relaxed field

Stein & Nordlund, ApJL 1989

Fraction of fluid reaching surface

Surface and Global Field Small scale flux is dominated by drag and is passively advected by the plasma flow. Only a very small fraction of fluid ascending from the bottom of the convection zone reaches the surface Much more flux and magnetic energy in the deep convection zone than at the surface. Energy added to flux that visits the surface is tiny compared to the global energy.

Why NO Surface Dynamo? Little local re-circulation. Magnetic field is carried to bottom of convection zone by downdrafts. Re-circulation is global, time scale is long Turnover time at bottom of convection zone is months. Energy in surface field is small Only small fraction of plasma starting up from bottom of convection zone reaches the surface.

Is there a Solar Surface Dynamo? Answer: No. There is a global dynamo process. Small scale fields are dragged and dispersed. Large scale fields are buoyant and feel the differential rotation and coriolis force.

The End