Initial configuration of the magnetic field. The vertical component of the magnetic field is plotted at the bottom of the domain. The two null points are indicated by small spheres. The field lines are plotted using a gray color map, which shows the magnitude of the magnetic field. The orange circle below one of the null points indicates the location of the velocity perturbation defined by Eq. (3), which has a center at the projection of the perturbed null point on the bottom boundary and a radius equal to $2\sqrt{{\sigma }_v/2}=0.447$ Mm. The isocontours of β_plasma=1 are shown by a semiopaque violet mesh of two regions, one encapsulating the two null points and an outer region.