Evolution of as a function of time for runs located on different parts of the S-curve: (top panel) 434P is a cold branch run, 468P a non-convective hot branch run, and (bottom panel) 442 a convective hot branch run. For the latter we show the simulation run with outflow (442O) and periodic boundary conditions (442P). We also plot, for run 442O, the contribution of the Maxwell stress to as a dashed line.