MHD Simulation of Flux Tube

A simulation with the FSAM code of the buoyant rise of an initially untwisted, uniformly buoyant toroidal flux tube.

Fan, Featherstone and Fang of HAO have used the finite difference spherical anelastic MHD (FSAM) code to carry out simulations of the buoyant rise of active region flux tubes in a giant-cell convective flow with a solar-like differential rotation.

FSAM Code
Simulation with the FSAM code

The simulations show that for buoyant flux tubes with an initial field strength of 100 kG, the magnetic buoyancy largely determines the rise of the tubes although strong down flows produce significant undulation and distortion to the shape of the emerging Ω-shaped loops. The convective flows significantly reduce the rise time it takes for the apex of the flux tube to reach the top. For the tubes with weak or no initial twist, it is found that the apex rises nearly radially to the top in about a month, and produces an emerging region (at a depth of about 30 Mm below the photosphere) with an overall tilt angle consistent with the active region tilts, although the emergence pattern is more complex compared to the case without convection. Near the top boundary at a depth of about 30 Mm, the emerging flux shows a retrograde zonal flow of about 345 m/s relative to the mean flow at that latitude.

Full caption for above figure: A simulation with the FSAM code of the buoyant rise of an initially untwisted, uniformly buoyant toroidal flux tube of 105 G. The top panel shows the volume rendering of the absolute magnetic field strength as the apex at the right is approaching the top boundary. Convection can produce loop structures with undulations that extend most of the depth of the CZ in a time scale of about a month. The apex portion rises nearly radially and produces an emerging region with an overall tilt angle consistent with active region tilts. The lower three panels show the radial magnetic field, radial velocity, and the zonal velocity at a depth of about 30 Mm below the photosphere. The emerging region shows an upflow of about 100 m/s, and a retrograde motion of about 345 m/s relative to the mean zonal flow at that latitude.

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