Time–distance plot showing the horizontal expansion of the spheromak along the x-direction along the line (y = 0, z _b  = 1.11) for four primitive variables: (a) ρ, (b) B _z , (c) v _x , and (d) v _y . The approximate average expansion rate of ﹩{dx}/{dt}\approx 2/1.4\left[{L}_{N}/{T}_{N}\right]﹩ is indicated by the white dashed line.