U-loop emergence in the Bifrost simulation. From left to right, there are three different times relative to t_b, a representative time for the emergence: (a) 100 s earlier, (b) t_b, (c) 200 s later, after the end of the relevant flux cancellation. The upper panels of each subfigure show the 3D field lines of interest in green, tracked by corks, and pale yellow field lines representing the core of the flux rope, above a 2D magnetogram. The view in the upper panels is angled toward lower y. The lower panels show the same 2D magnetogram, from directly above. The cancellation of interest occurs within the red circle.