LIC imaging averaged over the sampled time period of 30 min in the simulation. The location of the test vortices are indicated by a black cross at (x, y) = (0, 0) and black circles with diameters of D₀ = 200 km, 400 km, and 1000 km, respectively. The strong magnetic field region is indicated by a dashed contour line enclosing the region in which the time-averaged magnetic field strength is larger than the initial magnetic field strength of 50 G.