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Solar Surface MagnetoConvection Bob Stein, Dave Bercik - Michigan State Univ. Aake Nordlund - NBIfAFG • Flux Tubes ? • Small Scale Dynamo? • Flux Tube Evacuation? Simulation • • • • Snapshot of hydrodynamic convection Impose 1G horizontal seed field Top boundary: B -> potential field Bottom boundary: inflows advect 1G horizontal field Magnetic fields in intergranular lanes Probability distribution Magnetic Flux Observations Harvey & White 1999 Magnetic flux tubes Magnetic Flux Tubes Magnetic field lines Is there a Solar Surface Dynamo? Answer: There is a global small scale dynamo Flux Emergence & Disappearance Dynamo requirements • Magnetic field amplification by stretching and twisting • Diffusion to reconnect magnetic field lines and alter magnetic topology • Circulation to continue process Why NO Surface Dynamo? • Little local re-circulation Fluid flow little re-circulation Why NO Surface Dynamo? • Little local re-circulation • Magnetic field is carried down by downdrafts to bottom of convection zone Convective pumping Why NO Surface Dynamo? • Little local re-circulation. • Magnetic field is carried to bottom of convection zone by downdrafts. • Turnover time at bottom of convection zone is months. • Only small fraction of plasma starting up from bottom of convection zone reaches the surface. Fraction of fluid reaching surface Why NO Surface Dynamo? • Little local re-circulation. • Magnetic field is carried to bottom of convection zone by downdrafts. • Turnover time at bottom of convection zone is months. • Only small fraction of plasma starting up from bottom of convection zone reaches the surface. • Re-circulation is global, time scale is long Small Scale Global Dynamo • • • • Small scale magnetic flux Global circulation Long time scale (how long?) Seed field from dispersal of active regions dependence on solar cycle Next: follow field lines in time What happens when flux disappears? • Field lines Flux Tube Evacuation How do flux tubes become evacuated? Simulation • • • • Snapshot of hydrodynamic convection Impose 400G uniform vertical field Top boundary: B -> potential field Bottom boundary: B -> vertical Flux Tube structure - density Flux tube structure - temperature Stokes Profiles What’s next? • 30 G seed field, should produce larger surface field strengths and hopefully some flux concentrations that become evacuated (underway) • 500 G horizontal field advected in from bottom for a finite time to resemble an emerging flux tube (underway) • Supergranulation scale magneto-convection: 36 Mm wide by 10 Mm deep. The End