SSP seminar

Flux Emergence Simulations

Bob Stein (Michigan State University)

Monday 26th April 2010, 12:00pm
Pratt conference room, 60 Garden Street

We are simulating the rise of initially horizontal, untwisted, uniform magnetic flux from 20 Mm depth through the near the surface convection zone to the solar surface in a domain 48 Mm wide. These simulations are complimentary to the studies by Mark Cheung and Juan Martinez-Sykora of the emergence of coherent, twisted magnetic flux tubes. In both cases, the magnetic field is transported upward by diverging upflows aided by magnetic buoyancy, and pushed down by downdrafts, which produces a hierarchy of loop like structures, of increasingly smaller scale as the surface is approached. We compare two cases with field strengths of 5 and 20 kG at 20 Mm depth. The field initially emerges in a pepper and salt pattern of mixed polarities, but then collects into isolated unipolar areas. Elongated, enlarged granules develop where the flux emerges. In the stronger field, the average unsigned vertical field at tau=1 has reached 400 G with an rms of 800 G and a maximum of 4 kG. The average horizontal field has reached 350 G with an rms of 330 G and a maximum of 2.75 kG at tau=1. In this stronger field case, pores develop and where there are strong horizontal fields at their boundaries, bright edges develop similar to the hot wall effect in plages. In the weak field case, no pores have yet developed. In this case, the average unsigned vertical field at tau=1 is 43 G, its rms is 143 G and its maximum is 2.7 kG. The average horizontal field is 64 G, its rms 100 G and its maximum is 1.4 kG. In the stronger field strength case, the magnetic field significantly disturbs the convection below 3 Mm, inhibiting the vertical motion, shutting off convective energy transport and producing elongated cellular structures in the field direction. Shallower than 3 Mm the convection appears normal, but with concentrated vertical magnetic concentrations ("flux tubes") extending through the surface and producing pores where the field is greatest. Even in the weaker field case, the magnetic field inhibits vertical motions and convective energy transport, but now only below 10 Mm.


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