9.3 Active region flux emergence into the atmosphere

The evolution of active region scale flux tubes in the top layer of the solar convection zone, the process of sunspot formation at the surface, and the post-emergence evolution of subsurface flux tubes are not well understood (Section 8). However, rapid progress has been made in recent years in realistic radiation MHD simulations of magneto-convection in the top few Mm layer of the solar convection zone and the overlying photospheric layers. These simulations are able to reproduce many observed features seen in high resolution photospheric observations of the solar granulations, emerging flux regions, and sunspot fine structure (Section 8.1).

With regard to flux emergence into the solar atmosphere and the corona, it has been shown that the magnetic buoyancy instability is a mechanism through which magnetic flux reaching the photosphere can expand dynamically into the stably stratified solar atmosphere. Recent 3D MHD simulations of magnetic flux emergence into the solar atmosphere and reconnection of emerging flux with a pre-existing coronal field are able to reproduce many observed features in newly emerging active regions (Section 8.2).

These simulations suggest that a twisted subsurface flux tube does not rise bodily into the corona as a whole due to the heavy plasma that is trapped at the bottom concave portions of the helical field lines. Shear and rotational flows on the photosphere driven by the Lorentz force of the twisted flux tube during flux emergence are the crucial means whereby twist is transported from the interior into the solar corona, leading to the formation of a coronal flux rope with sigmoid-shaped, dipped core fields. The models suggest that sunspot rotations are a manifestation of nonlinear torsional Alfvén waves propagating along the emerging flux tube, transporting twist from the tube’s interior portion, where the rate of twist is high, towards the greatly stretched coronal portion, where the rate of twist is low. Driven by the continued vortical motions at the foot points of the emerged field lines, the newly formed coronal flux rope accelerates upwards and a current sheet of an overall sigmoid morphology develops. Heating associated with current sheet may cause the sigmoid-shaped field lines going through the current sheet to brighten up as the observed X-ray sigmoid loops in an active region (Section 8.2).

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